Rule J1.1(1) requires the OA to specify the dates of the event.  However, rule 89.2(b) requires 'adequate notice' when making changes to the notice of race.
Rule 80 lets the OA reschedule an event but the implication is that this is done before the event starts and not during or after the event.
Rule A1 requires the number of races scheduled to be stated in the notice of race or sailing instructions.

IMHO, I don't see how you can say that the OA or RC provided adequate notice for the changes to the schedule dates in the notice of race.  I think they can try but would have a tough time with any requests for redress.  I think that it would be easy to meet the requirements for redress in that adequate notice was not given.

I think that any competitor could easily seek and get redress if they are not happy with the change.  Even if they seem happy, I'd not continue unless I had documented consent.
Holding a race NOT in the timeframe set out in the NOR would, I think, easily be found to be either an error of the RC or, worse, a non-entity. Not a race at all. This could lead to problems with your regatta insurance.

I do recall a second level event where the format was lots of short one or two lap races, and everyone was having so much fun that after the last scheduled race the RC asked people if they wanted another race as they finished, and we all said yes. There might even have been two extras. But if it been a championship with the result hinging on the extra races I cannot imagine they would have survived a redress hearing. 

I think John C  has got all the relevant rules.

I'm not clear whether Antonio is saying:

Either way, I think it's a change to the NOR, and probably also the SI, so compliance with RRS 89.2(b), adequate notice for NOR, and RRS 90.2(c), for SI, is required.

The obvious problem is that by scheduling racing on a day outside the dates shown in the NOR, it may not be reasonably possible for some boats to race on that day.  In the second case, there is also the problem that the structure of the series specified in the NOR is being materially changed:  in other words, the series is not what competitors signed up for.

I'd agree with Michael that it would be wise to obtain the consent, or at least 'lack of objection' from all competitors.   This might be done at a competitors meeting or perhaps better, to start with, by posting a Notice to Competitors announcing the proposed change and inviting competitors to advise the OA of any objections, with a deadline.  Following this, any competitor who did not respond within the deadline, and then sought redress would be in 'fault of her own' and not entitled to redress.

I don't think RRS 80 necessarily requires rescheduling of a whole event to be done before the first originally scheduled date as long as the requirements adequate notice for changes to the NOR are met.  Possibly it may be necessary to abandon some races already sailed.

It's all a pretty big deal, and depends on doing what the competitors want.