You should probably borrow a fin to try before you buy one.
The bottom line is that the wave board will not go upwind as well as your freeride board. Moreover using the bigger fin you will loose some of the benefit of having the small board for wild bumpy conditions, the bigger fin will give you more lift and a bumpier ride at the same time.
I often jump off my slalomboard on to the wave board and initially try to sail it the same way. ie. crank the sail back and try to power off the fin...then I soon remember to adjust my stance, more upright, more weight on the front foot etc, as already mentioned, use the rail as an extension of the fin.
You may want to try the fin at the back off the box and then spend some time playing around with your stance.
Current fin ia a 211 Maui wavefin sanded back to 19.5cm..and a funny shape..I'll still have the crappy fin for when it gets really wild..
Just spend more time with the small fin learning how to sail with the smaller board.
My freestyle board feels completely different from my freeride board and i didn't like it at first and though I couldn't sail with my 16cm fin in so i used thje 24cm one it came with.
Now I can bust upwind as easily as on a freeride board with a 30cm fin.
Swing your weight forward and lean on the rail heaps, almost so your carving upwind to stop spinning out.
I can make my freestyle board spinout so easily from using backfoot pressure.
Guys I think it is largely the fin - is is a 21cm ground back to 19.5 and rough so it will be like trying to get your first waveboard upwind with an 18cm 
...... so new fin and technique not just the latter
IMO, as usual, Mark's advice is right on the money.
I find that the wave fin (I really like the MFC K-one fin) will suit the wave board better as it makes manouvres more smooth. When you gybe, the board will just flow round. The more vertical the fin, the more you have to steer the board round the turn.
The Freewave fins will be better upwind, but only slightly.
It depends much more on power. If you are well powered up, you will crank upwind easily. If the wind is light, it takes a lot more skill.
Only if you were using the waveboard a lot as a freeride board in marginal winds would I think it was worth getting the Freewave fin.
24cm Maui move ordered 
.. now I just need some decent wind! [}:)] ( sailing kind)..
