First time I broke out my 04 last season, I went 10 feet and heard a loud "Sqwueeeling shrill" noise, I thought the back brake was locking up, turns out the bearings were shot, sprocket side frozen solid!
Bike did not have THAT many miles, water must have settled in there over the winter.
I purchased a universal bearing puller (Pit Posse I think), as I didn't want to screw up the hub, searched this site for info, and did the job myself, no big deal. I think I used Pj's penatrating oil too. Putting the new bearings in the freezer really worked well on the install!
I pressed them in using a large threaded rod, from my bicycle headset tool. You can make one easily, find a threaded rod that fits the axel holes closely, use large flat washers (or the old bearings) and nuts that fit (duh), and tighten the nuts to pull it all in. Do one side at a time, easier to make sure the bearing is going in straight.
If you grind a slot (dremel) in the outer race of the old bearing, it won't get stuck in the race when using it for a "press" tool.
I recently used a Park Bicycle race removal tool to knock out the headset races on my KLX400. Park makes on for Motos. but the bike one worked too!
Park tool website has a lot of info, and How to section. Have fun!