I can't think of a USD fork that you can effectively change the oil on without removing them, there may be some I just haven't come across one yet (it's not saying much I haven't done stacks of them). To get all the oil out you generally need to disassemble the fork as well or at least pump the suspension repeatedly while out. The hardest part is lifting the front of the bike, after that it's a super quick job slipping the Rocket forks out. Just make sure you measure how much comes out of each leg! I didn't service my Rocket ones myself as I sent them away to get revalved but I imagine they would be similar to most other USD forks.