Do you have a backup of your DB that is old enough to still have entries for
the archives?
If so, DON"T LET THAT DB BACKUP EXPIRE!

I would try shutting down TSM, restoring the old DB, and doing an EXPORT of
that archive data.
Then I would restore the DB back to current, and IMPORT the archive data
again, specifying DATE=RELATIVE and a new policy domain with different
retention periods.

I"m pretty sure that will work for backups, I've never tried it for
archives.
What do you think, gang?  Would that work?