roccobike, I've not done that switch on any of my bikes, but I've known folks who've gone from 80mm to 100mm travel. No noticeable handling problems that I know of, just a plusher ride. Actually, I'm pretty sure I've seen identical frames that were available with 80mm or 100mm travel, depending on the specific model of the bike.
What you might do is check the axle to crown measurement of the two forks and see what the difference is. It might not even be 20mm. One thing about it, you'll probably set the longer travel fork up with more sag than the shorter fork, so the net difference won't be a full 20mm even if the axle to crown dimmension is 20mm different..........This is making my brain hurt. I think it will be fine.
