Merge branch without common ancestor (and make it the new parent/merge branch)

I have a branch lp:~blueyed/b2evolution/dev, which is derived from lp:~blueyed/b2evolution/trunk-cvs.
The trunk-cvs branch is maintained by tailor, but I want to use lp:~vcs-imports/b2evolution/trunk as parent now.

The contents of those branches should be identical, but merging (via "bzr merge lp:~vcs-imports/b2evolution/trunk -r0..8145") adds a bunch of conflicts only (since bzr handles files internally by IDs, not contents AFAIK).

So, the only way to change the branch to another parent appears to be merging, resolving the conflicts (by manually adding changes from e.g. "blogs.moved" to "blogs"), then commit this?
(Alternatively, I could create a patch from the dev branch (against trunk-cvs) and apply this to the merge result).

Anyway, all the history from the dev branch would be void (at least in the current files).

An alternative might be to branch "trunk" (the new parent), and apply the changes from "dev" therein.
But this would mean that derived branches from "dev" could not get merged anymore (since then they do not have a common ancestor anymore).

Any suggestions?

(Question #68312 is related - has something happened in this regard in the last months?)