You should be able to set the four trim commands to four separate buttons so reassigning them should be easy.
My key file looks like this:
SimTrimNoseDown 0 -1 -3 0 0x0 0
SimTrimRollRight 0 -1 -3 2 0x0 0
SimTrimNoseUp 0 -1 -3 4 0x0 0
SimTrimRollLeft 0 -1 -3 6 0x0 0
If I wanted to exchange nose up and down trim I would make it:
SimTrimNoseDown 0 -1 -3 4 0x0 0
SimTrimRollRight 0 -1 -3 2 0x0 0
SimTrimNoseUp 0 -1 -3 0 0x0 0
SimTrimRollLeft 0 -1 -3 6 0x0 0
Then loading that key file should apply the change. You should be able to do this simply in the game interface though. POV hat directions are 0-7 value. 0 is up, 1 is up-right, 2 is right, and so on around clockwise until 7 is up left.