MIB Agent names appear to be assigned based on the basis of first name. Where two agents share a first initial, a second initial or a descriptor can be used to disambiguate.
In MiB3 we meet J's new partner "Double-A" (formerly Andrew Arnett). This presumably leaves the way open for there to also be an "Agent A"
In MiBI, we meet "High T", formerly Agent T. This presumably leadsleaves the way open for there to be a new Agent T.