Army Ceremonial Band Bass Trombone?

I think the Ceremonial Band carries three bass trombones. One retired and one was promoted into an administrative role.

With very few exceptions, with the Army Band once you're in you're in. I haven't ever heard of anyone in the Army Band being separated for artistic reasons.

I don't have knowledge of this particular situation, but the military ceremonial bands often carry a fairly large complement of musicians, including multiples of the same instruments, so it's very possible they just have a new vacancy. It's because the band almost never plays all together, the musicians are divided out into smaller ensembles to play Arlington funerals and other diplomatic events as needed.