into forms that make them appear as though they have always been based on the new upstream git repositories

We have always been at war with git-svn. Oh no, wait, we have never been at war with git-svn. :-)

(Seems sensible enough to me. I've just been figuring out how to do the same thing to the more convoluted of my old informal git-svn checkouts using git filter-branch rather than reposurgeon...)