Two main hurdles:

The later style mirrors will not fit with the exterior window trim that sits above the door panel and will have to be removed. And you will need to get the rubber trim from a doner vehicle unless you want water in the bottom of the door!! It just prys up with a bit of care.

Removal of the interior "trim" of the later mirros is a PITA since it has a friction fit socket which loves to break off upon a pry removal and a tab that sits belwo the top of the door panel.

The interior door panel must be off to remove the trim, and with the window down, you can push the trim retainer out of the doors sheet metal.

Other than that it is a remove and replace as long as you get all the screws, brackets, retainers and trim of the doner mirror. The holes are already in the 85's door for the updated mirrors.

I got all the parts for my 85's upgrade from a 90 in the salvage yard for $38.

Took about 10 minutes to remove everything.

Duane