I am not sure if you still need this info.
There was a thread going on about this, and a workaround was suggested.
The workaround was to change certain config that currently allows Unified Manager to store 390 days of data, once that config is changed the Unified Manager server has to be restarted. If you are aware of this, pls try that with caution, as the suggested workaround applies to entire data retention policy of Unified manager database, and not only to a specific table related to snapmirror transfer history.
Excuse me for not posting the workaround here.
-Thx
