Are whirlpool rough-in-valves universal or are they specific to manufacture like a shower rough-in-valve? I would like to change out the trim on our whirlpool. We currently have a Kohler faucet and valve and would like to change the trim to a Delta faucet. Should I be able to just take the Kohler trim off and replace it with the new Delta trim or do all the internal parts, including the valve, need to be replaced as well?

You will need to replace all of the parts.

If you are trying to just change the 'look' of the bathroom, there may be Kohler trim that closely matches the look you are going for that is compatible with the present rough-in valve.