The Olds I use actually played very well in tune naturally, even the low F (the E...not so much), I pulled the 2nd slide about half an inch, and the third slide maybe a cm or two sometimes. The main slide was out a ways though because I only liked to use one bit. If it's well enough in tune for the big band I play in, it's definitely good enough for marching/pep band.
The Olds has a lot of pullable slides: 1st, 2nd, and two for 3rd so no conversion needed (or would have been used for that matter; it's marching band). I never had to pull slides but I imagine I could have if I didn't have to hold the neck up.
1905 Boosey Class A Euphonium-Wick SM4M
Yamaha 301M Marching Baritone-Schilke 52
1960 Conn 11J-Conn Helleberg
1961 Conn 14J-Vincent DFL
2015 King 2341-Bach Corp. 24AW
Olds O95 Sousaphone-King 26