I too am finding much of what I read in this thread infuriating, but for different reasons.
Far as I can tell, not everyone is suggesting that Miranda doesn't have the right to a day off. Not everyone is suggesting that Munoz is a less-than-capable substitute (having seen him, I can attest to how excellent he is). Not everyone is suggesting that the producers don't have the right to make this decision, whether in March when tickets went onsale, or a month or two from now.
Where everyone is mistaken, though, is in thinking that the producers really don't have a moral obligation to garner goodwill and do the right thing by, at the bare minimum, offering direct refunds at this point to those who have a ticket for a Munoz peformance and who choose that option. (After all, as has been suggested here, they will surely not have trouble reselling them at face value, at the bare minimum.)
As I've stated above, I paid to see the show off-Broadway (at already inflated off-Broadway prices, late in the run), and found out the day before that we would not be seeing Miranda. Fair enough. We went. We love. We thought Munoz was terrific. But then we bought tickets to see it again this fall specifically to see Miranda. Obviously an illness would always have been a possibility, and we might have been stuck seeing the understudy again, but this isn't that. And personally, I WOULD suggest that if this didn't seem a little bit underhanded - or choose your own word, sneaky, suspicious, shady, etc. - then they wouldn't have a problem offering up such refunds. We're out of the country this week, but when we return you can bet we'll be calling Ticketmaster and, having seen the show once already with the understudy, insisting on it. That isn't 'entitlement' - that's just decency, fairness, and common sense.
Honestly, the truth is, if the producers don't do as they should and direct Ticketmaster to offer those refunds or exchanges, then - just as people who can't see Broadway shows live often find an, ahem, alternate way to view them - there's a way for resourceful people to get that refund AND stick it to the producers at the same time by making sure the seats aren't paid for and can't be resold. I took advantage of it once about 30 or 40 years ago, and hoped I'd never have to again, as I must admit it's a bit underhanded and should be altogether unnecessary. That said, if the producers and Ticketmaster don't do the right thing, then we'll have no problem lying just a little bit to make sure that the right thing in fact gets done. And I guarantee that we won't be alone.
Hopefully the producers will see the light and change their approach on this matter before we have to stoop to that level.