When a 401(k) plan is amended to add SIMPLE provisions, would prior profit sharing or match contributions have to be vested 100%? Would you be able to keep the prior contributions under the plan's vesting schedule and use any forfeitures that may occur to reduce the contribution deposit to be made for the SIMPLE match or SIMPLE ps contributions?

Are you talking about trying to amend a qualified plan into a SIMPLE Plan? If so, that is not possible. The current plan would have to be terminated and everything would be 100% vested. The EEs would have the option of rolling into an IRA.

An existing 401(k) plan may be amended into a SIMPLE 401(k) plan pursuant to Revenue Procedure 97-9 via a Model Amendment.

Well, I did not know that. You learn something new everyday!