I run all 3. You could perhaps get away without GFO when running pellets, but pellets are more effective against nitrate, so, I find GFO to still be necessary. Pellets really do not do anything similar to GAC "charcoal", so, I don't see why you'd replace charcoal. Actually, bacteria can produce some toxic byproducts, that should be effectively removed with charcoal, so, i would suspect it is more important when running pellets. You may actually want to increase the charcoal IMHO.
As to switching the GFO to pellets, if you go that route, I would probably get a good test kit such as the Hach or Hanna 736 ULR phosphourous checker and monitor. It takes a while for the pellets to kick in, so, perhaps wait a few weeks, then remove 1/2 the GFO, then wait another few weeks and remove the rest if PO4 is still undetectable.