It uses the same battery (~0.90 Wh, yeah tiny) as the earlier Oasis, so probably a good guess it uses the same process to replace the battery (spudger to pry it off slowly I believe).
Just as a FYI, the Oasis originally launched with a battery cover where the main battery was kept (~4.5 Wh), but I'm guessing they had problems with that and dropped it and so the current Oasis has this tiny little battery in it (without telling potential customers of course). The Paperwhite has a ~5.0Wh battery.