Migrating IMAP (nnimap) servers?

[Bruce Stephens]

Suppose I read email using foreign nnimap groups to some server, and
that I know the server's going to be changed.  What can I do about
that in order to improve my chances of not losing stuff?  (Let's
suppose that if the administrators offer to move existing email, that
they can accurately move IMAP tags and things.)  

The manual covers changing nntp servers, but I don't see any similar
advice on moving IMAP servers.  Is that because gnus doesn't really
cache much, so it all just works?

(I guess one thing I'll do regardless is make copies of email in nnml
folders, just in case.)