I've just replaced the filter on my 20v with a K&N. It is reasonably simple to fit, you do have to remove the battery and battery carrier to get at the box. It's best to soak the three bolts holding the outlet pipe from the box with a freeing agent before you start. I found one of my clips holding the intake pipework was in fact a nylon tie - a trip to the local accessory shop followed so it's best to do the work when the shops are open!
Beware of the self tapping screws holding the box cover, the heads are small (8mm socket size) and they do have a tendency to rust, if you can't remove them you have a problem as they have to be drilled out.
Lots of threads on the subject.
p.s. the filter box does not have a swirl pot fitted.