I'm thinking about creating a modcomp which will auto-delete all units that are too obsolete, so to speak. For example, when you can build Macemen, all your old warriors will be deleted, unless you upgrade them when prompted. When I can build muskets, archers will be deleted, and so on. So, I want a pop-up box when I can build Macemen, that will ask me to spend gold NOW to upgrade all warriors, or they will be deleted! This may sound harsh, but I would like this feature anyways. :) Does anyone have an idea on how to approach this?

The finding out which units are obsolete might just be sorting each class by strength, and then if the list is 4 units long, kill off the bottom unit. The length of death would depend on the mod, so it could be pretty simple for basic civ, and rather complicated if you want this to work for any mod.

You could have a Medieval unit that costs the same as Warrior and fights almost the same as Warrior (iCombat=3) yet is among the units Warrior can upgrade to. Some kind of levy with pitchforks or something. Or just give Warrior a MiddleArtDefine so it doesn't look like a classic caveman. But then you want a popup box. Sounds like an event.