The "trick" is to remove as narrow a line of plastic as you can, if you just want to reposition the kit part. You'll need to thin the trailing edges of the wing, stabilizer, or tail.
Then you need to replicate the part of the control surface that is now showing. Depending on the scale & which control you are moving, you can use strip or sheet styrene or even stretched sprue.
I dropped the flaps on the A-36 in my signature pic. I used the backside of a #11 blade, thinned the trailing edges with files & sandpaper, and used sheet styrene for the new exposed surface.
Hope this helps.