Historical Novel question about posting

[James Ryan Writer]

My novel is set at the time of Augustus, 5AD, to be precise.

My question is this:

I have a Centurion who is badly injured in battle and is sent home to Rome to recover. Once he's pronounced fit where would he go to be reassigned to a new legion? as he can't return to his old unit.

Any help on Roman military administration in Rome would be appreciated.

Thanks.