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Has it been done? Could it be done without sacrifices?
I guess there are experts in this excellent forum that can answer these questions.
I am aware of that CC sousas has been made, the ones I have tried both had some funny intonation issues though.
Why you may ask, well, I am a CC-fanatic...

Even in US$ 20K may cut you a decent CC.

As for cutting circlophoniums this only can be done if they have a loop which can be removed. BBb Conns of the current wrap have no such loop.

Joe S cut a BBb Buescher sousaphone to a CC helicon. It started out with the old-style huge U-leadpipe including the main tuning slide. There most of the length was removed. I think that Joe also was smart in choosing the new bell in a way allowing to get some shorting there also without introducing a step in the bore.

Such project takes a genius repairman with great 3D-imagination and a workshop with a steady flow of funny parts (that flow also goes through the bay). And then it takes countless hours.

The 20K does not start out with an ideal intonation even in BBb. If the likelihood of a successful conversion of that bore progression was there, we would see some CC BAT's made out of 2XJ branches. Such CC's don't appear to be common.

Klaus Smedegaard Bjerre

This would require making some of the inside bows from scratch - sheet brass. I do this from time-to-time, but I have to charge accordingly - there's a bit of labor involven in making and bending bows. The valve chops on this model wouldn't be to horrific.

If you're interested, PM me and I'll look at one in my shop with that eye. Basically, it'll be a bit different player, since there's only a couple places to take the plumbing from.

J.c.S.