Phixtit,
If your just putting a 21" wheel on then 9 out of 10 times you just bolt it on and go. Sometimes you may have to "oval " out the mounting holes on the fender and tip it either back or forward for clearance. Here is my 2008 SERK that I had to "oval" out the holes.

If you want to put a 23" you can do it a few ways. 1. Cut the neck with an HHI kit and and raked triple trees. 2. As mentioned Pickard has a set of trees out now that are proven. 3 Trike trees. Most people buy a fender for a 23" but some have used the stock one, IMHO stock one sits too high and looks awkward.
If you want a 26" wheel you have to cut and rake it as well and a new fender is needed. There is a bolt on set of trees that does not require cutting the neck but IMHO it just does not look right. The bike sits way to high and its all rake in the trees nothing in the neck. When cutting the neck and bolting on the HHI kit it moves the neck 1.5" forward and 1" up and you get 44 degree rake. Basically its not just all raked with the trees. It looks proportioned.
If you want a 30" cut the neck and triple trees are the only options at this point.
Example of cutting the neck and HHI kit and trees on my bike.



