It seems like Git could change the disk format eventually with backwards compatibility for current SHA1 addresses. You don't even need to rewrite old objects. You just need to know which hash method a given tree/commit object is using to verify history or perform checkout. That could be a flag or tag in the object.
Git has some work in progress in that direction, starting by changing all the internal data structures to use a struct rather than a hardcoded array of bytes the size of a SHA1.
