Upgrading units once completed in my Capitol?

Engineer, medic etc.. I'm having trouble repeating this because IDK how it happened and I'm having trouble finding related threads here, thx..

I noticed after my archers were completed in my capitol, I had the ability to immediately upgrade them using the triangle button while they were still in the city before I moved them. How did I do that? I've been going over the tech tree and found nothing and it was really early in the game. Thx for the help.

Civ Rev I, not II

Are you playing Germany? As I recall, their starting bonus is the ability to upgrade elite units.