I posted a question in a TSM discussion group regarding how to change the retain version of the data that has already been archived.
Here is my question: I archived some data last year, and the retain version was set to 365 days. Now I want to keep those archived data for a longer time, say 700 days. How can I do it? I have updated to retain version to 700 days in the the copy group archive policy. My understanding is that it only impacts the future archive, but not for the data that has already been archived. Any suggestions or comments will be appreciated. Thanks!
Here is the best answer I think: Unlike backuped files, archived files are never rebound to a different management class. If you change the management class for a file, any previous copies of the file that you archived remain bound to the management class specified when you archived them.
At best, you can “suspend” expiration on archived files using event based retention. There is a post by Tade Nate (dated 2008) explaining how to do that :
http://nocoast-tech.blogspot.fr/2008/06/rebinding-tsm-archives-so-they-do-not.html
[Updates]: Some one points me to the right place page 493 of http://pic.dhe.ibm.com/infocenter/tsminfo/v6r3/topic/com.ibm.itsm.srv.doc/b_srv_admin_guide_aix.pdf
Archive copies
Archive copies are never rebound because each archive operation creates a
different archive copy. Archive copies remain bound to the management class
name specified when the user archived them.
If the management class to which an archive copy is bound no longer exists or no
longer contains an archive copy group, the server uses the default management
class. If you later change or replace the default management class, the server uses
the updated default management class to manage the archive copy.
If the default management class does not contain an archive copy group, the server
uses the archive retention grace period specified for the policy domain.
Jackie Chen's IT Workshop 
Dear Jackie
Thanks a lot for all the work you do to bring us a little knowledge about TSM, now, if you can, help me with this, I got requests and I don’t know how exactly answer them, they are many, but 2 are the most important. First, how can I assign a tape and tell it to store the backups from january an july, so when I’ll have to restore those data I can tell: mount tape 4 there’s the backup of march 2014″, and take the tape off from the library once the restore is complete. Is there some way that I can send data to a tape from one specific date or is it always with nex storagepool?
My second question comes together with the first. How about if in the next 3 years I want to restore a backup? what is the best practice to do that? only the retention option o for long term there is another way?
Again Thanks a lot for all the effort you do posting all these tutorials for TSM.