I'm with you on this, J. The Quarter Inching valve is about the only thing about the B7610 that I don't like.
A month or so ago someone on the forum had a new B7800 that had problems and Kubota didn't exactly jump right on it, as I recall, and the buyer was subjected to some suffering. As part of a deal to make things right, Kubota offered to convert the B7800's quarter-inching 3pt. control to Position Control. I noticed it when the buyer asked on the forum whether it was wise to go ahead with the PC conversion. From this I concluded that it IS possible to upgrade/replace Q-I with PC.
Another obvious clue is that the B7510DTN narrow vineyard tractor has PC....although I guess it is possible that the DTN has a special transmission casting with oil passages missing from the B7510DT and HSD.
From the lack of response to your post, it's obvious that it's rarely or never done.
Bob