Hi Jan,
you are completely right. As you mentioned, possible (and I mean correct
too) way is 1. change retention periods to reflect your new policy
requirements, 2. validate policy set, 3. activate policy set, 4. run
expiration on TSM server manually (command EXPIRE INVENTORY) and 5. reclaim
free space on volumes (or see your volumes goes SCRATCH if reclamation runs
automatically). Change of retention periods affects all files binded to
management class you've changed, this means previous backed up as well as
future backed up files.

Hope this helps.

Tom