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. 

It is also very common on the San Francisco City Front when there is a flood tide on any given weekend.

First, I'm staying out of the match racing and team racing discussion as I have been a competitor and trained for it but never been an on the water judge and there are experienced judges on this forum who are already discussing the related issues above.  Just dealing with W/L issue in fleet racing.

In fleet racing this is a W/L and mainsail position judgement.  There are many dinghy one design fleets that sail significantly by the lee, they are usually cat-rigged or main-jib boats, no spinnaker.  Some boats such as the Laser can sail 25 degrees by the lee if trimmed and balanced correctly and the competitor retains starboard rights while doing so.  This can be observed in national, international, and olympic level events with on the water judges present.  Since I believe the original poster was discussing fleet racing and not match or team racing, I believe the relevant information is the definition and the rule below:

 

I asked this question here in 2019, there were three answers that Rule 11 is determined by mainsail position as defined in the definition W&L (see link below).  Since then I asked a pair of on the water judges at a national level Laser event and they confirmed this.
Traditional W/L (Rule 11) or does the OD fleet racing style matter?

Paul,

Could you explain how you work that out? 

Rule 23.3, relevant parts accurately quoted by Phil above,

So Last sentence doesn't apply to me exawe are discussing. 

How is proper course relevant? 

Umpires might adopt and publish to competitors an umpire policy like the following.

If a boat has not been penalised by an umpire or clearly indicated by hail or gesture that she will take a penalty, then umpires will not treat her as a boat taking a penalty.

If umpires were really concerned about the sportsmanship aspects of this coached play, they could adopt an umpire policy like the following, but it's very heavy handed and may be contentious.

If a competitor reaches out and deliberately touches another boat with their hand, other than to fend off a boat that is failing to keep clear, the boat may be penalised for a breach of sportsmanship