Trikester has done the 17" rear wheel conversion. I expect he'll be along shortly : )
Ah, how right you are. I'm too damn predictable!
Yes I converted the rear wheel so I could use the Kenda K270 tires front and rear. Just like your situation, I used a rear tire on the front and it worked great. 4.6"x17 on front and 5.1"x17 rear. I used that big front tire because I wanted floatation in sand.
Changing the rear to 17" was a big job. I had to cut a curved section out of the swing arm cross tube brace (no easy job, believe me) and then weld a 1/8" aluminum plate in there to regain strength and close the opening. The 2013's have essentially that same thing formed into the cross tube brace in order to come from the factory with a 17" wheel. I also had to remove part of the shock mount and turn the bottom damper adj forward.
I originally did this on my 2010 DS. When I bought the 2012 DS I swapped swing arms in order to avoid having to go through all of that again, so the original 16" wheel is back on the 2010 with a 2012 swing arm.
I'm very happy that they finally changed on the 2013's and even went to an 18" rear and 21" front on the FX. I have K270's on the FX with a 3.25" front and 5.1" rear. I did have to change the rear rim from the 2.15" width to a 2.5" to accommodate the wide rear tire.
Trikester