If they're like the mounts for MK2 (same cabinet design, so a good chance), you will either have to modify them or fabricate new ones. I just fabricated new ones when I installed my 27" so that I could keep the old ones in the event I wanted to swap monitors back out for a 25".
Depending on your new monitor frame, you may just have to cut off the mounting studs for the old 25" size and drill holes for the new frame if you elect to modify the existing mounts. Sometimes it's also necessary to slice off a little of the existing metal to get the new frame to fit.