This is interesting as the replies cover replacing the barrel / breech block as a unit. Whenever I see a 'barrel replacement' post I automatically think of just replacing the barrel proper i.e. the tube the pellet goes down, not including the breech block into which the barrel is fixed.
I must assume, therefore that if someone wished to swap calibre from, say, .177 to .22 on a break barrel the job would just entail swapping the barrel/breech block plus, most likely, an enlargement of the transfer port. Maybe an adjustment to the mainspring as well.
Cheers, Phil