Ask United. They did such a migration.
Or VIA, they recently replaced their core reservations system with a modern one, though they also call the new one ReserVIA. It was very late and at one point they pulled an already late implementation less than 60 days before the publicly announced implementation date. But they finally got it implemented earlier this year. Even then it wasn't smooth, but it's up.
From what I've been given to understand it is a somewhat customized version of a European rail reservations system. It wasn't built in house, which is seldom done in any type of industry or organization today.
I've been involved in a few core system replacements in my career and they are huge endeavors, are extremely fraught and difficult, take a lot of time and money, and are usually late. The easiest, which wasn't easy, the actual migration was done by a gradual "sell over" to the new system, with both the old and new system running side by side. It took five years
after production implementation to complete the sell over. Far more difficult is a straight up conversion, converting all current operations and recent data in a big bang (you convert and load as much data as you can beforehand). Usually over a weekend with all IT staff on deck and biting their fingernails. That's what has to be done with a common carrier's reservations system. You cannot run two reservations systems at once for one carrier.
I am happy I was able to wait to reserve this year's VIA trip until after the new system was implemented, so my reservation didn't have to undergo conversion. I know something of how that sausage would be made, and didn't want to eat it.