Welcome to FT!, cubed02

Once the trip is finished and if the second leg does not clear, you will be able (by calling MP customer support, not reservations) to have the difference between a domestic upgrade and the international upgrade refunded.

As a non-elite the folks you will be dealing with may not be the best and it may take a little effort -- but it is standard policy.

You will have to pay the international miles and co-pay at the start of the trip. And then request the refund.