By freezing the DB plan, you can elect whether it is offset by future additions to the MP plan. You do end up with vesting issues, but it is probably moot. The DB still provides a minimum benefit until distributed. We have used floor offsets which only use "new money" in a new plan or separate account of an existing plan. Similarly, we have used floor offsets that only use "old money" that ignores new accruals. But you could certainly use the entire DC account as an offset.
With over-funding, you could also increase the participation span or benefit levels of the DB minimum plan to use up a portion or all of the excess assets, assuming you meet non-discrimination rules.