I'd think that the switch from button to keyboard is easier than from PA to CBA. Is there a reason to do this? Of course, there is always a reason for anything, and nothing is sacrosanct or forbidden, however that is a question that each person needs to answer for themselves.
The advantages of moving to a CBA are offset by the approximate 10 year transitional process for anyone past the beginner stage. If you are intermediate or above, its not unreasonable to spend a decade working to get back to where you were, so this is not something that I would suggest for anyone over 50 years of age... imagine the struggle of working (wasting?) 10 years to reach where you are today only to find that you are now in the part of the process where you are sliding back down instead of upwards?
As far as the other way around, short of personal preference, a physical issue (missing a thumb, its a LOT easier to get on with a CBA than PA) or perhaps availability, I'd probably say that it takes maybe 50-75% of the time to migrate over to PA from a CBA, There are a LOT more CBA players that can play on a PA than there are PA players that can play on a CBA, at least within my own experiences, though none ever considered moving over to PA.
This is one of those questions that has only one perfect answer... "IT DEPENDS...".