Hi Lair.
Yes, CMOD calculates which documents are eligible for expiration at the time that maintenance is run, using the value from 'Life of Data and Indexes'.
HOWEVER... If you're using Tivoli Storage Manager, and you've set a specific retention period at the management class level, you ALSO have to change that setting, or data will be discarded by TSM based on it's own configuration. As a result, if you have other App Groups that share the same TSM-enabled Storage Set in CMOD, they will need to be moved out of that Storage Set, because they will have their retention extended as a side effect.
-JD.