ok - before I do any of this I want to be sure I am dealing with the right files......
(my name)/library/mail/mac-myname (note: the last is not an IMAP)
there is also a file: CalDAVfirstname.lastname@example.org
I believe the first is the one you were describing but the second?? do I archive/compress both? or only the first?
Whew, 10.6.x makes it difficult because it assumes any .me. mac addy uses the old servers, you may have to make the new IMAP account some frivilous nane like email@example.com, wait for it to fail at every step, then after creating the account, go back & edit everything to the correct parameters.
No idea on CalDAV, none here,but I haven't trusted Apple in a decade or so with anything important.