fsteers, I'm not sure you need to change the octave, remember that treble clef Bb for trombone in brass bands already sounds an octave lower than written.
As a trombone player myself moving between bands, orchestras and big bands I'd say the following:
For a trombone player who reads Bb treble clef (ie brass band clef), to play a piece in tenor clef just read it as if it is in the Bb treble you usually read but imagine there are two more sharps (or two fewer flats) in the key signature. So what you would see as treble clef F and C need to be imagined to be a semitone higher than written. Tenor clef is used for trombonists as a way to write music that is reasonably high and would be difficult to read in bass clef due to ledger lines so you are reading bass clef moved down a few lines.
The third space note in tenor clef with a flat is a concert B-flat - played exactly the same as a third space C in brass band Bb treble clef on trombone.
If you were changing it into treble clef to play on a piano in concert pitch it would be necessary to change the octave (but not the key signature). Yes, confusing if you haven't had a few theory lessons!
In brief again if I've lost you - on a trombone, play tenor clef exactly as if it is in brass band treble clef but remove two flats from the key signature.
Hopefully makes sense! If not let me know and I'll jump on Sibelius and write an example for you.