It's hard to get out of the habit of holding the horn the same way- same hand and hand position. Not every horn is the same, though!
I play a ton of different trombones, and it took me a while to figure out what my body was telling me, and why sometimes one wouldn't work.
This seems stupidly simple, but it has helped at least a couple people... just rotate the trombone to a different angle to keep the mouthpiece on the face the same way.
Here's me holding one of my rotor basses, and one of my axial basses:
![Image](https://i.imgur.com/OA0ZtLB.jpg)
Now, I will say there's no way to deal with a slide that's too narrow or a valve that leaves no room. I couldn't play my Bach 36K because the valve and slide width just didn't match, even with my smaller size neck. I'm sure there are plenty that can't play the Conn 30H or 32H width.
But if you run into unfamiliar horns with different ergonomics... just rotate. Let the mouthpiece mesh with your face the usual way and change the arm instead.