That's pretty much how the instructions recommend it's done - not removing the trim completely is explicitly stated.For the skeptics - didn't completely remove the trim. Just propped it up, put a protective rubber mat underneath and then cut with a Dremel (fiberglass re-inforced cutting tool). Once the initial cut was made, I could easily cut the rest of the trim into the prescribed lengths. Super easy. Took about 30-45 minutes per side.
Cutting it in place, I read as, not even popping it out of the gutter.
Anyway, good going.