Perhaps Equity will make an allowance for you given the tough situation, but just going by the book, Rule 10 (C) seems to indicate that no actor can work more than 8 days without a break, period. It looks like an exception is made for SMs, allowing us to pass on the Designated Day Off for a price, but this rule creates a problem for the 1 actor you’ve got who hasn’t missed any rehearsals. I think you’re right that the sick days are just that, and not days off, and if that’s the case the sick actor is in the same situation as the other actor (but the book doesn’t spell that out). If Equity does let the actors work anyway, I sure hope they get paid 2/6th!
This rule also says you can’t change the Designated Day Off without unanimous consent of actors, and with not less than one week’s notice. The week’s notice part would be the second problem here, if you’re talking about this coming Saturday. We can’t decide retroactively that because Actor 1 was not called to rehearsal on a previous day, we can now tell them that a day which has already passed will now be considered the day off, even though we didn’t know that originally when we decided not to call them. That would set a pretty crazy precedent. If Equity makes an exception here, you’ll also have to look at all your future scheduled days off to make sure they’re at the right intervals.
You’ve probably already thought of this, but if the director is only trying to squeeze in three more hours of rehearsal, another possibility is to keep the Designated Day Off where it is, and then tack those three hours onto the first work day of the following week. All 5 Equity folks would need O/T pay for it, but it might be worth it since it is a small cast. Good luck...and happy birthday wherever you spend it.