I'm betting on a simple inversion - those currently spared will be required to sacrifice themselves to allow Thanos to be defeated, and to allow the others to return.
Downey Jr. ain't committing to a new series of films, and Hemsworth peobably neither. Way to go out with a big bang.
I'm getting to the point where I wouldn't rely on contract rumors to determine where the story goes. They may have signed NDAs on future contract agreements and such, and thus any talk about not renewing might be smoke and mirrors.
Besides, what you suggest is way too obvious, and I'm hoping for something a bit more creative. However, some sort of universal reset via the stones would be a good way to do some recasting, if they feel they must.
Remember that Tony Stark did actually die in his comic line, then some weird thing with the nano-bots of his suit brought him back. What's to say he's going to look 100% the same?