Change upgrade

Is there a way to change what upgrades affect units during the game?
For example, is it possible to write a trigger that makes a knight become affected by the defend upgrade if it has less than 50 life?

yes... but not with triggers, your going to need jass and weeader's bonus mod system. With that you can add armor to units mid game.

Thanks for your reply fugly, but In my post above, making the knight become affected by the defend upgrade was just an example, it was just to make what I wanted to do more clear. Is there a way to change what upgrades affect units during the game?

I don't like double posting, but this seems pretty dead. Does anybody know the answer to my problem?

You can remove the actual ability, but you can't change tech requirements on the fly.

The easiest way to get the desired effect will be to fake the entire upgrade system.

You will need to use a system of units which can be trained (and appear to be upgrades) which when trianed are removed. Then use abilties to reproduce the effects after detecting the trianing of such units. The system should be flexible enough for the effect you want.