You don't require a change to the axle or hubs. The new 15-inch wheels will fit the existing lugs. FWIW, the hole pattern is called "5 on 4 1/2" and it is pretty standard on RVs, campers, boat trailers, utility trailers, and so forth.
If you need a lift (you don't necessarily, but probably) it is quite easy to add, even if you choose to do it yourself. For example, see Bill Miller's post in the TM Technical Library.
http://www.trailmanorowners.com/foru...ad.php?t=12132
Any shop that deals with trailers can handle it. It does not require a TM dealer, or even an RV dealer.
Bill