You could probably move the fin head back some, but I do not think you will get the change in performance your are looking for.
If the fin trails far behind the board, my current opinion (after many hours of sailing
the old 50 degree fins and many more hours sailing True Ames SB Weeds, some special vario rake SB Weeds, many sizes of the Tangent Dynamics vario rake weeds, and most of all Wolfgang Lessachers assymetrical Duo Weeds) is that your best and least expensive way to improve the performance (sailing performance, not weed shedding performance) is to purchase one of the new generation of weed fins.
Changing the angle of a weed fin really does not work as the foil is generated on the
LE angle. Change the LE angle and your foil no longer matches.
Vario Rake LE's are really the best as they shed the shallow surface weeds, but do not trail behind the board so much and the more vertical LE angle deeper on the fin gives you better performance both upwind and speed.
If you really want a super performing weed fin, try a Lessacher Design Duo (Assymetrical foils) Weed.
You can cut the head off and move it, but I do not know how you will restore the strength. I would not want to sail a weed fin with a moved back fin head very far from shore.
Sorry, I just don't think this is a good idea when new generation weed fins do not cost all that much.