gday Quack,
Doing a similar swap on an 07 deuce-using a spoked v-rod wheel.
I had to swap bearings because I'm using cvo twin disk sliders but ,if u use a late model dyna wheel u may find wheel bearings are same size probably 25mm ,but could be 1"-early model possibly 3/4"-not a big deal -all ball bearing wheel bearings are the same O/D-51mm -i think only I/D & bearing width changes between 15mm & 21mm-the wheel u get will determine bearing width-remember to double check the center spacer size -there have been a lot of issues & a few posts with wheel bearings-mainly dyna's -i believe they only like about 2 thou clearance.
U will need disk spacer-I used Vulcan engineering-great service ,well made product ,great shipping prices -highly recommended-I'd put the link on ,but don't no how.
They have various widths ,so once you have your spacer size ,pick the closest & then machine to size (if necessary)
The other way is to buy a 19"smoothy rim ,19" spokes to suit & re-lace -there was an excellent post on that subject-plus you can use stainless spokes & alloy rim -minimal rust????
The other thing will be tyre size-as you will be able to fit 100 x 19 or 110X 19 (which is a fair bit wider)-I can't say which is better -have had both on an fxr -but if compatable with rear tire, I would probably go the 110-speek to the tire guys-opinion's are bit like arseholes -everybody's got one-I just try & go with what the majority recommend-
Maybe some one will have some recomendations??

I want to do the same to my 08 fxst and i thought that I could just get the smooth rim only and re-lace my fxst hub with stainless spokes, end rust problem. Is this an option or not.

Just read the above. well I dont have to have a long neck to be a goose

Why do you want to get rid of your 21" rim . Before I got my fxst I had a Yamaha XVS 1100 with an 18" front and it was a shit of a thing ,it tracked in every rut and groove, the 21 floats over all but the worst surfaces.