Adjusting mana cost of a multi-level hero ability

I've seen this done in another map, but since I don't know the steps to get around its map protection, I'll just ask in here...

How can I create a passive ability that lowers the mana cost of another ability? There is no Trigger Action for altering an ability's mana cost - not even in WEU (or not that I can find). I also looked for an Upgrade Effect to do the same thing, but all Upgrades can do is change the spell level. I tried to set up a trigger that would replace the original ability with an identical version that just has a reduced mana cost, but this didn't work because it's a multi-level hero spell, and using the "Unit - Add Ability" function doesn't allow the usage of skill points to train it. Is there a JASS work-around for this, or possibly a GUI method that I'm just overlooking?

Well this is an easy one...
Ok, have u heard of the Engineering Upgrade ability? it changes abilities from one to another. For a better example look at the Goblin Tinkerer. So heres what you do:
1 - Make the ability that you want the mana cost lowered of twice. Once with the normal amount of mana, and one with the lowered amount.
2 - Make another spell based off Engineering Upgrade, and change the data on it, so that it replaces the first ability with the new one that has lower mana.
3 - Give the ability (your engineering upgrade) to the hero, and when the player researches it, it will replace the other spell, with the new one that has lower mana.
Understand? it's pretty simple. I hope that helped...and if it did..plz add some reputation...Thnx.