MEP as successor plan... but requiring rollovers into the MEP

The sponsor of a PEO MEP is bringing about a new client that already has their own 401k plan.  It's a small plan, but very messy, with provisions that are not part of the MEP (e.g., vesting, distribution options).  I'm thinking that we can't tell them to terminate the 401k and then join the MEP - that might run afoul of successor plan rules.  But what about if we terminate the 401k and tell all the participants that they have to rollover into the MEP?

Or is there another way around this (other than amending the MEP to allow for all sorts of strange things for this one adopting ER only)?

Thanks.

Could they freeze the current plan and start a new plan in the MEP?  It would be expensive, complicated, and difficult to explain to employees so it is not a desired solution.

A general question, can't an adopting employer in the MEP choose some of their own provisions, which differ from the MEP master plan provisions?  For example, would there be a benefits rights and features issue if different adopting employers choose to offer different provisions - adopting employer 1 has no hardships, 2 loans, and installment payments and adopting employer 2 has hardships, 1 loan, and lump sum distirbutions only?