Ken Ruth/Bald Mountain does the premier work on this. He does full feed and take up conversions and a feed-side-only conversion - load 120, use 620 takeup reel, less difficult job. I don't know the cost difference, but the typical job on a Medalist there is supposedly >$400 now. It's time-consuming too.
Option B is Manfred Schmidt in the Chicago area. $120 + shipping for feed-side-only conversion, about $90 for CLA. Probably $200 for combined job.
For CLA only, Carol Miller at Flutot's in California is highly recommended on APUG, roughly $50-60 for CLA only. I don't think she does conversions.
Lens design was made specifically for the Medalist. Same design was allegedly prototype for first Hasselblad. Urban legend has it Hasselblad switched to Zeiss after WWII not to improve quality but to reduce cost.
Many people reroll 120 onto 620 spools. I have never mastered that.