Dealing with rewritten upstream

I have a repo w/over two years of history whose upstream repo is a git-svn mirror.

The upstream folks recently announced they need to retire the existing repo and replace it with a new repo. The new repo is identical to the old repo tree wise (commit for commit), but some of the commits in the old repo had incorrect authorship which is corrected in the new repo, so the new repo has different commit IDs than the old.

(i.e., it's as if they've run filter-branch --env-filter on the old repo.)

My repo has many merge points with the old history.

Pictorially:

---A---B---C---D---E... new-upstream/master

---a---b---c---d---e... old-upstream/master
    \       \       \
     1---2---3---4---5  master

The obvious way do deal with this situation is:

$ git merge -s ours -m "Splice in new-upstream/master" E

Are there any other/better options I'm missing?

(Eventually upstream plans to migrate entirely to git, so I can't just run git-svn myself.)

j.