I do not know if the extra 20000 miles this membership year can be considered for renewal the next year, but it cannot be used for qualification for the next tier - at least this was the case for me. If OP has 100000 miles the next membership year, I don't think the extra 20000 miles this year can be added to give him a Diamond status. If that is OP's intention, then the miles may be wasted.
Personally I have not tried the trick of removing the MPC# and adding back later, and I don't know if I will ever try that. It sounds like it has been working for people.