ATC doesn't always use the same callsigns as appear in GDS systems or are used on FIDS, RIDS, GIDS, apps, etc. When I worked in SOC for UA, we had lots of transcons that were renumbered on day of departure to avoid confusion with other transcons. We would mostly use 8xxx numbers for those flights. So for example, To keep "United 555" from being confused with "Delta 555", it would be filed as say UA8105. This is transparent to basically everyone but ATC, dispatch and the pilots. Example here of why this is important:
As I said way back, generally on round robins, a letter or something is appended to the end of the callsign if it gets stubbed (meaning both segments are in the air at the same time or in the wrong order). There's tons of hacks that go on like this behind the scene to make it easier.

As others have mentioned, adding a character would be a significant work effort (something akin to Y2K or Y2K38) and have minimal benefit.