From: ADSM: Dist Stor Manager on behalf of tsmadmin account for Excaliber 
Business Solutions
I would like to know if i have a retention period for 180 days for Archives in 
my archive copy group.
I then change this retetion period after a  20 day archiving process later to 
30 days, will the data I have archived
for the past 20 days take the new retention period of 30 days.

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(This one's going in the FAQ.) No. Once you create an archive, no future act of 
any kind will affect the amount of time that archive is retained. The only way 
to change an archive's retention is to retrieve the archive, and then rearchive 
it with a different retention rule.
 
--
Mark Stapleton