So far there are subscription export and import tools. However for posts and comments there do not exist any atm.

I'm trying to think of a way that would not be too resource intensive, and within the spirit of "not re-writing history".

I think if you had to move, you would want to try to make it irreversible (once the feature is stable enough), lock your old account and it will link on the profile page to your new one.

This is how I might implement it: During the move process, the original canonical link could stay attributed to your old account handle (perhaps with a note saying "account moved" in the same way there's an indicator for a banned account), no other changes to the original post. Then under your new profile your old aliases are registered, and old posts appear under your profile with your handle replaced by something like this: "user@newserver as user@oldserver". There is a chance that links to posts on defunct servers or ones your new servers are deferated with might be broken or missing. Being able to locate your old handle's posts using your new handle in search would be difficult but that could come as a feature later.