I've checked Joe Straczynski's notes and can't find anything, so this answer s speculative.
Lt Corwin is, by this point, the longest-serving member of the operations staff. You'll note that in this season, both Dr Franklin and Security Chief Zack Allen continue to wear their 'reborn' uniforms from their time serving under Captain Sheridan as the head of an independent Babylon 5. In the first episode of season 5, Captain Lochley has a conversation with Corwin as she assumes command of the station, in which Corwin clarifies that B5 will remain an independent state. Sheridan, who was technically the legal head of state for an independent B5, specifically requested an Earthforce officer to take over his position as station chief, but there are still officers on board the station who serve an independent B5.
Corwin, like Zack and Stephen, is one of these officers, and as such he wears the uniform he wore after Sheridan broke away from Earth. All the regular crew kept their Earthforce uniforms during Sheridan's coup, but as an officer (officially he's the station's second-in-command by the time of season 5, and was in charge during the period between Ivanova's transfer and Lochley's arrival) Corwin chose a different uniform during Sheridan's rule. It doesn't match what the main staff wore (something that always bothered me; I liked Corwin, and thought he deserved a 'rebirth' uniform himself) but it is internally consistent with the series. You'll remember the brief period between Sheridan removing his Earthforce uniform and receiving a 'rebirth' uniform in season 3, where he also only wore a dress shirt while on duty.