TSM purge are a great domain for questions.
Like Justin said, it depends.
You have basically 2 ways to use TSM with CMOD:
1) You have storage node in TSM configured as RETINIT=EVENT, then TSM will not purge the documents/objects, it will wait that CMOD sends the event to delete that document/object. This is really in sync with CMOD and with arsmaint
2) You have storage node in TSM configured as RETINIT=CREATION, then TSM will purge the documents after the period RETVER days is passed. In that case, TSM and CMOD are working in an asynchron way, and arsmaint will only remove the entries in the library server and never in TSM.
Now, if you are interested in converting your RETINIT=CREATION storage node in RETINIT=EVENT... then with TSM you cannot, it won't work. The only way I found to do it, was to export and reimport each TSM object from one storage node to the desired storage node... that's not that difficult, but takes a long time, and need to be careful not to loose the CMOD link...
I'm waiting that you can do that in TSM in a standard procedure!!