Hi,
I'm considering an upgrade of my kona paddywagon to front disc brake. In order to do so, I'll need to replace my stock Kona Retro Road fork to something like THIS
My main concern is that stock fork has 380mm axle to crown when cyclocross one is 398. Would it change my frame geometry significantly? Also, any critique of the project idea or other ways to implement it would be appreciated.
Thanks,
-Igor.

Well, significantly means different things to different people. I would probably notice the change - but not be troubled by it. But I'm sure that other people would swear blind that a 20 mm diff in axle-to-crown would cause so much of a change to make the bike virtually unrideable.

Oh, there's rake too to consider.

Yes, you are right. The problem is I can't find the rake data for kona fork on internet. Nashbar one is 45mm.
Edited:
Actually, I've find additional data for kona here:
Fork Length: 375mm
Fork offset (I'm assuming that is the rake): 43mm

So, how bad is it comparing to nashbar 380mm and 45mm?
Thanks,
-Igor.

I don't think anyone would be able to tell those forks apart in a blind test. For the axle-to-crown it's in the ballpark of running a wider tire at the front, and for the rake - forget about it.

Igor, With a full set of instruments you'd see a difference, seat of the pants difference will be nil.

Brad