I want to add an adjustible fuel pressure regulator. My question is, do I have to reprogram the ECM to inform it about the higher pressure, or will it think something is wrong and compensate for it?

 

What you are trying to do is fool the computer into delivering more fuel. Under normal operating your computer will recognize the richer mixture and take measures to shorten the duty cylce of the injectors. However at WOT you will be delivering a little more fuel.