Steps to assessing this redress claim under RRS 62.1(a): 
1. Was there an improper action or omission?
2. Was the boat's score made significantly worse as a result of the action / omission through no fault of her own?
 

1. Was there an improper action or omission? 

If the SI is to be interpreted in the way you suggest, it is effectively establishing the ends of the starting line as marks that boats must leave on one side or the other in order to sail the course. If following this logic, then shifting the ends of the line would need to be done in compliance with RRS 33. I have inferred this was not complied with. Therefore, shifting the ends of the line without signalling a change of course was an improper action of the race committee. 


Counter-argument: 

What should the wording 'are restricted...'  be interpreted to mean? Seems very unclear to ascertain the meaning or instruction from that SI. If there was a protest concerning this rule, I'd be inclined to set aside the SI and dismiss the protest because of its vagueness. So arguably the SI did not have the effect of making the ends of the line marks, and as boats were free to sail either side of the line, the race committee were entitled to shift them as they please. Therefore, no improper action, and redress denied. 

 
2. Was the boat's score made significantly worse as a result of the action / omission through no fault of her own?
This would seem to require a factual enquiry to establish. It would depend on when the race committee shifted the mark and how far ahead boat A was of boat B. Did one boat have more of an opportunity to see the change of position and adjust their plan accordingly? Furthermore - how far were the marks moved? Could Boat B's gains be attributed to taking the short cut, or were there other factors at play. You mention Boat B did not pass until 'sometime later', which would suggest the latter. 

I will never understand why classes and/or RCs set themselves up for this situation.  It's too easy to avoid.  Regardless . . .

I would not consider the line buoy a mark when not starting or finishing.  The definition of mark includes the requirement that a mark/object/vessel be left on a specified side.  In this scenario, you can pass the line either to port or starboard.

RRS 33 doesn't work here - you must signal at the previous rounding mark the direction of the next mark.  What would you signal?  Only one end is moving - and by my other argument, it's not a mark anyway.  I think that would be confusing to competitors - they would think the leeward mark is being moved.

I'm having a hard time finding that this was an improper action, as much as I would like to do so.  Ill-considered does not necessarily mean improper.

Edit: US Sailing Question 118 does give some insight into the meaning of "improper" in the context of a race committee's actions.

The restricted start/finish line must be assumed to be fluid.
Otherwise the RC would be forbidden to adjust the line for later starts in the event of a wind shift.

I would definitely give redress. To give a boat an advantage over another by changing a restriction is an improper action. I do not like these courses. Si could control if correctly written 

Mike,

My understanding of the OP was that as boats were approaching the line, the race committee moved the pin in order to shorten up their finishing line (which is fairly common).

One boat (or most boats) sailed close aboard the pin buoy in its new position and one boat sailed around the position where the pin buoy previously was.

You're saying that some boats passed outside the pin buoy in its original position then the race committee moved it, and some boats passed close aboard it in its new position.

In that position, wabsolutely redress.

Supposing that the crazy situation, described in the two preceding posts, of the race committee moving the pin buoy while some boats in the fleet had rounded and some hadn't, does NOT apply, and we just have a boat rounding the hypothetical position of the buoy in the starting line position, here are my thoughts.

"Restricted"
The word 'restricted' does not have any meaning given to it by the RRS.  Objectively I have trouble placing any meaning on the SI.

If I was a visiting judge and the matter came up in a hearing, and two local judges said "everyone around here knows that that means that boats must not sail through the line", I could accept that ... UNTIL a party comes into the hearing and says "I have no idea what 'restricted' means ... "

If you want to do this, your SI need to do two things:

State "boats shall not cross the line between ... ... "  and
State "the line between ... ... is an obstruction.

Designating the line as an obstruction prevents a right of way boat shepherding a boat across the line then protesting them. 

Shortening up the line

The SI refer to  "the Start/Finish lines".

Somebody, like the boat requesting redress might contend that the Start/Finish line was a single line, and that changing it's extent (or position?) during racing was improper.

I don't think that this is a correct construction of the phrase.  The phrase refers to "lines", plural, IMHO it refers to Starting lines and Finishing lines, and contemplates that they will be different, and, as Phil has suggested above may be in different positions or orientations.

Furthermore, I don't think the lines need to be laid all the time.  Suppose that early starting divisions are approaching the starting/finishing area while later divisions are still starting:  they will be required to not cross the starting line, but there is no finishing line in place so they have no obligation about a finishing line.

As I see it, what happens is that, once the last boat has started, the race committee lifts the starting pin:  at this point the starting line no longer exists (and if there were any boats approaching the starting/finishing area, they would have no obligation about the starting line or the finishing line.

The race committee then re-lays the pin buoy in position as the finishing pin, and the finishing line comes into existence and boats will have an obligation not to sail through it.

Note also that under the 2021 rules, depending on what the SI might say, the display of orange or blue flags might affect whether a starting line or a finishing line was in existence.

If the race committee performs the re-laying process in such a way that the transition from starting line to finishing adversely affects a boat, then that would be an improper action, but if the race committee accomplishes the re-laying in ample time for all boats to see the buoy in its new position, then I don't think there is problem.

These issues may be avoided by changing the manner in which the cognizant SI is written.  "During the starting sequences, the starting line shall not be crossed by a yacht except during her starting period or starting.  The finish line shall not be crossed while the blue finish flag is flying except when finishing."

Dick Rose wrote an entire column in 2010 about the proper wording to use instead of "the start-finish line is closed" (whatever that means). Aside from rule wording changes in the last 11 years (!). His comments remain valid.☒

Neither Theodor's nor Clarks/Dick Roses wordings address the problem illustrated in the diagram in the Dick Rose article of a right-of-way boat shepherding the give way boat across the line and protesting.

The line needs to be designated as an obstruction in the SI. 

Regarding the original question of moving the pin on a "restricted" line - unless the line is unreasonably long and the course is unreasonably short, moving the pin even by many boat lengths will probably only shorten the rhumb line track by a few feet or yards. And given that boats seldom sail the rhumb line downwind anyway, I don't think the difference in distance sailed between boat A and boat B caused by the change would be significant compared to all of the other factors at play.  

So even if the PC decided that moving the pin constituted an improper action by the RC, I'd say redress would be finishing places.

Designation of obstructions in SI is a more recent issue than Dick Rose's 2010 article and would undoubtedly be incorporated in a re-issue of the guidance.

An Australian Sailing Appeals Committee has clearly expressed an interpretation that when an area is 'designated' in the SI as an area into which boats shall not sail, this designates that area as an obstruction without further words.

2021-01 WMSC Sassy vs Javelin