I answered this on the other board as well, but maybe it will be helpful:

I don't know if you can have your MAC on different active donation numbers simultaneously. Here is how I would try to shift it if this is the case:

1. On the donation you are leaving, modify the assigned MAC to something else
2. Assign the MAC to your new donation (Don't assign a MAC to this donation until you have to.)

Here is Gabacho's response: Use the old donation number to enter the Mac form and delete it. Then use the new one as usual to enter in the form.