I did this with my "Bessophone." When I bought it years ago, it had the detachable Miraphone recording bell. Tuba Exchange had fashioned an upright detachable bell out of a St. Pete to go on it as well. Well, it was too short and TubaTinker extended the legs of the main tuning slide about two inches per leg to compensate, and did a superlative job for me. After a few years, when I decided I preferred the tone of the Besson bell, my local tech and I measured, and measured and measured: height, diameter at the ferrule, etc., for a couple of weeks before switching. It turned out perfect. We did it during winter break, and my tech was happy to have a "real" job to work on instead of just fixing school horn damage or the steady diet of re-rounding and fixing guides on Yamaha valve casings (his words - not mine).
Now, here's the deal:
1) Measurement is King (pun intended.) You must make sure the EFFECTIVE length of the bell is the same, because of the terminal node that is different depending on the different throat geometry and rim diameter of different bells. Sorry, I don't know the math or rule-of-thumb to get there (maybe Sam Gnagny or Matt Walters know); all I know is that on mine, the slightly narrower throat combined with a 1/2 inch larger diameter rim on the Besson New Standard BBb 17 inch rim balanced out the slightly larger throat but only 16 1/2 inch diameter rim of the St. Pete bell.
2) The leadpipe. If the lead pipe is solidly soldered to the bell, it is going to be a &!+ยข# to unsolder. It takes lots and lots of time and care, slipping in card stock that won't scorch and keep solder from re-flowing as you work the leadpipe loose. Then you have the problem of an unsupported leadpipe that could very easily be kinked or otherwise damaged if care is not taken to keep it stable in the interim.
3) Installation of the new bell, assuming all the geometry issues have been properly addressed, is straightforward. My tech dry-fitted it, but initially it was not good. Then we realized that out of abundance of caution, he had trimmed it long. So another quarter or three-eighths inch off, and it slid into the stack ferrule like a glove, soldered up clean, and it did exactly what I wanted. Then he repositioned the receiver brace. And because of the slightly smaller throat on the Besson bell, I got the bonus of a suspended leadpipe that improved intonation and response, especially first ledger line first valve Eb, which I had assumed was a leaky worn first valve rotor, but turned out to be the detachable tenon and collar was in the wrong place on the effectively shorter upright bell, and impeded the anti-node in the throat.
Bottom line (second pun intended), and especially after it cleared up intonation quirks and resonance issues, if you are sure you don't want to change bells again, I agree with going forward with the switch. It turned my tuba from being just OK, even above average, to being perfect for my application, my embouchure, and my playing style and repertoire. I receive ongoing compliments as to the foundation it provides for the ensembles I perform with.
Recently, a friend of mine borrowed it to play a brass quintet concert. I attended the concert. That was the first time I had actually gotten to hear it out front. WOW! I will put its tone and intonation up against any tuba in the world, from a $100 garage sale find, to any of these $30,000+ "custom" tubas. It's core, breadth, and foundation were even better than I had hoped for, and of course you can't tell sitting under the bell exactly what it does out front.
One last item: the H. N. White company were historically very particular about matching bells to horns, what we call "mechanical impedance," to make sure everything functioned together properly and that intonation and response were even throughout the range. That also affects the choice of mouthpiece. If you can't find an old pre-cyborg King 25 (slightly shallower) or 26 (deep and dark) mouthpiece to bring out all the tuba will have to offer when you are done, I have sold all my other mouthpieces except for a Blokepiece Imperial with a trimmed spacer and the 32.6mm modified helleberg lexan rim. With the choice of shanks, bowls, rims and the spacer that can be modified, you will find a Blokepiece combination to provide the perfect interface (again, pun intended) between the embouchure and the tuba (yes, I know - there is no such thing as a "perfect" mouthpiece, but you're reading a post from a guy who has tried several dozen over the years and sold all but the Imperial and a Kelly lexan for outdoor extreme weather).
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