If when you get to Rank 5 Level 20 and you want to switch your abilities around.
You lose 1 level to do so right?
So does that make you Rank 4 Level 19 again or are you still Rank 5 but Level 19?
Do all of the Rank 5 abilities get locked again?

you go back to rank 4 i beleive. i am basing this off when I reset my abilities at level 15 and went back to Rank 3.

You do lose ranks when you delevel.