Seniority...
Marty,
You bring up an interesting question. Mach E. A. has some good points. Though at the end, I think he forgot a critical item he had up front.
More seats equals more crew required. Not just for the transition, but for the long haul.
Part of the useful background is missing. How many aircraft? Crew? What's turnover like? Etc.
The airline industry is based on seniority. The Regional jet Captain becomes a 737/A320 FO. That's normal. It's OK for the "left seat" (only) pilot to move "up" to the right seat of something bigger. Happens all the time with night freight or similar in the US, from sole-pilot to the coveted CRJ right seat.
The complicating factor here is you need to hire experience you don't have, but the people you hire don't have the specialized experience your existing crews have. How long does it take to learn the fundamentals?
The existing employees have corporate seniority over the new hires. "Anyone" can pass the jet's type rating in a couple weeks, but your existing hires have the "outback" knowledge that takes years (?) to develop. So, what's fair?
Mach said something about "training themselves out of a job." Probably not true since you're doubling the number of required crew.
Everyone involved needs to understand what they bring to the table and that each group is expected to bring their expertise to the table and share. If they'd rather die than learn from someone, then they know where the door is.
Put the upgrade pilots in the right seat. Pay them the same as the new hires in the left seat. Tell everyone they have six months to learn what they need to know. After six months they are expected to be rosterable as Captain or FO. And use them in rotation that way. Anyone who can't/won't do that flies only FO. New hires are FOs with eventual upgrades. You only have uncertainty for six months.
Barely related historical bit that has value. German created an Army from scratch in 1955. They filled an entire structure in one go. There were men who were Colonels for the next 20 years. There were also men who were Captains for the next 20 years. Eventually some Colonels retired so some Lieutenant Colonels could get promoted, letting some Majors move up, letting some Captains move up. As they neared retirement, the Captains were promoted to Major, then to Lieutenant Colonel for a few years so status caught up. There wasn't much difference in pay, otherwise lifetime earnings would have been unfairly different. Now, 65 years later all that has long since been worked out.