Someone mentioned doing this yourself.
Back in the "very old" days, ARC sent out kits to us dealers for upgrading the SP-3 to an SP-3A1. I did about a dozen for my customers back then. I forget exactly--it was 48 years ago--but as I remember it was about 10 parts. HOWEVER, upgrading to the D-76A amp was a COMPLETE change inside the case, so it had to be done at the factory.
I would guess your Ref6 upgrade needs the factory people to go through it and make sure everything is right. It is expensive, but if you love the new sound, go for it. As the saying goes, “The bitterness of poor quality remains long after the sweetness of low price is forgotten.” ― Benjamin Franklin
Cheers!