Regarding (1), not "everything will work as before".
What happens if a Fossil repo that has had SHA3 commits written to it is accessed by old Fossil software before that change was introduced?
What happens if a Fossil repo that has had SHA3 commits written to it is accessed by old Fossil software before that change was introduced?
If you have an old clone made from before the transition and try to update it, I'm not sure what it says, since I don't have any of those around any more. It has, after all, been three years since Fossil began to move on this problem, so that it's largely a past issue for us now.
This transition time was indeed annoying for us over in Fossil land, but Git's going to have to go through a transition like this, too. The question isn't whether but how long we'll have to wait for it to begin and how long it'll take to complete.
The moment I can't read new repos with an installation of git 1.6 or 1.7, I'm ditching the garbage and finding something else.
Forward and backward compatibility, forever, please!
Beyond about 10 years, you usually end up freezing old binaries in place along with old data in order to continue manipulating it anyway.