From what I recall, they said that you can do it whenever. Whenever you log into a character that has more progression in altars and map discovery than your account is at currently, your account progress will be updated.

It should. I believe the way it works now, whenever you find an altar on one toon, whenever you log into another toon they should sync up.

Basically it should works like mount or other premium skin. So one character unlock it all other characters will also got it

Quite a treasure you have there in that horadric boob