You're actually lucky with this.
Many folks who have the A-A K-Line sets in this second generation (in Yellow/Black Boxes) are trying to upgrade their controls and face a quite a bit of work as a result. The A-A's as a set have 4 motors, which is more than most upgrade electronics can handle.
Since you only have one, i.e. two motors instead of four, you shouldn't have as big a problem.
Start with this link. It describes what the upgrade consists of, ERR Cruise Commander, which you'll need, and modifications to it to handle four motors, which you won't:
How To: ERR Cruise Commander With Tethered Bridge for 4-Motor Operation | gunrunnerjohn
You'll learn a lot about the K-Line A-A sets and hopefully enough to guide you on your single A upgrade.
Mike