Currently running a GFO Reactor and a Charcoal reactor in a 125 gal.
Will be switching over to a Bio Pellet Reactor once I get a larger skimmer set-up.
Should I go cold turkey when I switch it over or do the switch slowly?
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.
I'd recommend going cold turkey. I'm actually 4 months into biopellets with no results, but I've been running GFO at the same time. My phosphates have been at zero, but nitrates are higher. I talked with Avast Marine and searched around on here and it was recommended to remove the GFO because both need to be present in order for the biopellets to get started. Removing GFO and carbon isn't going to cause any ill effects for in the time it should take for the biopellets to get going.
When running bio-pellets remove all gfo media. You don't want the gfo to compete with the bacteria growing in the pellets. Dosing Microbacter7 every week is very beneficial as well. I'm now moving to dosing MB7 daily at the rate of 2.5 ml per day and see if there are adittional benefits to this. I have a 125g/35g sump. Best of luck !