When moving up in valve size from 1.84" to 1.94" or from 1.94" to 2.02" is it necessary to have new seats installed or can the old seats be re-ground to the bigger diameter? I've heard conflicting opinions on this.

 

depends on the heads. Smaller chamber ones going up big steps, like 1.72-2.02 would probably need new seats pushed in. Otherwise, if you cut the current material, your valve will be sunk deeper into the chamber.
Going from 1.84-1.94 doesn't require it.