You need a larger mount if you swap the entire mirror, however you can open the mirrors and swap the mounts so you don't have to replace the windshield.
Someone has reported using lockwire to saw off the old mount. It's the wire used to hold bolts and nuts in place on race prepped cars. I believe the diameter was 0.25. Supposedly the wire broke a few times and took time but was successful. There are definitely more than one way to skin this cat.

For those that are not as well prepared to make harnesses I really do recommend you buy the harness from Kufatec. A lot of their harnesses are sometime a bit short but it's easier to lengthen a wire if needed than to make an entire harness.