It's a similar problem with 'Like' -- how do you 'like' a post about someone grieving a loss? FB is adding a different reaction button soon.

I think switching from Favorite is a good thing, as favoriting feels just personal, whereas a 'like' or a 'heart' is a public vote of support/appreciation. I do agree, though, that 'heart' is odd, but I can't think of anything else better.