I see that my rather 'tonge in cheek' comment got a not too undeserved response ...... and maybe that Jack's Band had an excess of Cornet players. To be honest the practices and attitudes within bands can differ quite a bit between bands; the movement that you talk about is certainly something that I've never seen - outside of learner bands where the bulk of people start on a Cornet- but that's not to say it doesn't happen in a significant percentage of bands. Though, in some cases, inflexibility and self-centredness is an issue it can actually be a quite understandable one. It can take some people literally years of practice to become reasonably able on a particular type of instrument (e.g. Cornet). Additionally they very likely have invested not in-significant money to buy their own instrument, accessories, special practice material and not in-significant time to properly understand of their band pieces and to develop friendships/comradery and to gain credibility within their current section. The shift to a different instrument can involve, at least partially, the loss of all that investment and maybe leave the individual feeling quite vulnerable. It can take further years of effort to regain ones past level of skill and expertise on the new instrument. Of course (simplistically) the more skilled and experienced the musician the easier the transition, indeed such a person might be returning to something that they played a couple of decades back and so pick things up (again) relatively painlessly.