I'm fairly experienced in the BPCR game, but I'm not a gunsmith. Don't even play one on TV.
I won't say it can't be done (reline), but it's certainly not common. I'm sure there are technical issues, including the new caliber. Possibly a small, low pressure round. I do know that .22 RF liners are quite common, including removable inserts allowing .22 RF to be shot in a big bore BPCR.
Most just bite the bullet and have a new barrel fitted.
Paul