Raising Hero abilities with Upgrades

I'm making a TD and I'm trying to give a tower a hero ability that you can then raise the level of by researching an upgrade at a Home Base tower. It works pretty good for all the towers I make before upgrading, but if I upgrade all the way and then build a tower, the ability is only level 2 instead of 3. Does anyone know how I can fix this?

You'd have to use an event that checks if a building was finished, a unit type comparrison to see what kind of building, and then an action setting it to the proper level.

So you'd have to keep track of what level the upgrade was at prolly with an integer variable. Like whenever the player upgrades, set the integer variable "upgradelevel" or whatever to arithmatic addition ("upgradelevel + 1)