This was not that big a deal on most railroads... the locomotive number referred to the machinery portion of the locomotive (the "engine" if you will), and boilers were, well, boilers. Most roads actually replaced boilers on a regular basis. For example, I've heard that some PRR K4s locomotives had three boilers during their lifetime, replaced as each successive boiler wore out. Since the PRR had very a very large shop well experienced in locomotive design and construction, it was probably easier to build a new boiler than rebuild a worn-out one. I've even heard that one K4s (pacific) got a swapped out boiler from a L1s (mikado)... it was a Lima-built mike and it supposedly retained the Lima builders plate once it was put on the K. (Lima did not build any K4s-class locos... most were home-built by Juniata, but the last batch (5400's) had some members that due to heavy shop demands were built by Baldwin).
Bill Daniels
Santa Rosa, California