I'm going to go back to the OP and try and answer his original question.
The plates are called cassette cogs. The cassette doesn't necessarily have to come off. BUT, if you're trying to clean and regrease a traditional cup and cone hub (as opposed to a sealed cartridge bearing) to lockring that holds the cogs in place must come off. Once the lockring is off you might as well take the cassette off because if you don't the cogs will probably fall off in a bit of carelessness and you'll have the unpleasurable task of putting the cogs and spacers back in the correct order. Better to take it off all together and set it aside so everything stays in order. Once that's taken care of you'll have to loosen and remove the locknut and cone from the non-drive side. Hopefully at this point the axle along with the driveside locknut and cone will be able to drop out the driveside. A bunch of bearings will probably fall out as well at some point in this process, don't lose them.
Once the axle is out you clean everything up, pack with new grease, and put it all back together. The trickiest part is setting the bearing preload when reassembling. It's not hard but there's a bit of finesse to getting it 'just right.'
To do this you'll need a cassette lockring tool and cone wrenches. If you want to do it yourself and learn how come visit the bike co op.
If your hub uses sealed cartridge bearings - take it to a shop. The old ones will have to be driven out then new ones pressed in. Depending on the manufacturer, it might be really easy or it might be a major PITA.