Originally Posted by queerpunk

if you need a 150mm stem, then the fork might not be the part of the frameset due for a replacement. but it sounds like you're working on that.

the Langster Pro uses a pretty standard 1 1/8" steerer tube.
it comes stock with a fork with 45mm rake. you can probably safely replace it with a fork with 40mm of rake, too, without experiencing anything weird.

however, going down to an Alpina, which has 28mm of rake, would change the handling quite a bit - perhaps more than you're used to, and depending on the size of your bike (the LP has different head tube angles through the size ranges), could get it firmly into "this is bad" territory.

in order to put another fork onto your bike, you take it to a shop, and they pop off the crown race off of your old fork (that's the part of your headset that's pressed on to your fork), hammer it on to a new fork, and pop the new fork in. It's simple and straightforward.