Most noticeable effect will be needing a new, longer-reach front brake. Not necessarily, but likely with such a large change in rake. If it has canti posts, they might be in the wrong place afterward, to where no brake will work.

I use a soft rubber mallet. Use the largest one you can find; the size means the force is spread out over a large portion of the tube, not concentrated in one place.
I set the crown and dropouts on a piece of softwood, with a strip of rubber over it if the paint is precious.

Soft whaps at first, with measuring to see if you've moved it. If no movement, whap a little harder, check again, iterate as necessary. Once you figure out how hard you have to hit it to get it moving, hit it in more than one place so you take out rake all through the curved part. It will be eaiser to unrake at first, the first couple mm, but the force required will go up a bit after that.

The way you "measure" to see if you're moving it is setting the crown and dropouts on a flat plane (e.g. benchtop) and see if both sides of the crown and both dropouts touch at the same time. If the crown is touching both sides but only one dropout is, that's the side with more rake. Check that before you start; don't assume the fork was properly aligned.