Why all the character switch ups?

Actor availability I'm sure. After the first one was made I'm sure they just called up and see who they could get. Then wrote those characters in accordingly.

And yes, I realize this post is a few months old, but you never know. You might still check the page occasionally :)

So I hear Dads dead, hey is that eggnog?
-Dave Foley

It depended on who was willing to come back. Maybe Bruce Mahler, who played Fackler (that's his name) was asked to come back and he decided to do it. He didn't have a big part in the third film, by the way.

There's a few reasons why this happens. Actor availability, as someone said.
Some actors may get asked to do a movie but have another project their working on and can't commit to more then one at a time.
Some may not get asked due to statistics of their character not being "popular" enough to watch. Other actors (this is a hypothetical example) may have become big movie/tv stars (there are a few here) and wanted to much money. (Again, they was hypothetical)
Some actors flat out decline also.
...The Asian actor from the second one was in a couple Revenge of the Nerds movies btw in case you're interested in seeing him again...

"The Asian cadet from the 2nd movie he was in part 3 but never heard from again."

Cadet Nogata played by Brian Tochi only appeared in Police Academy 3 and 4.

'When there's no more room in Hollywood, remakes shall walk the Earth.'