How to change manacost in game

Hi,
I try making a spell which decreases the manacost for a target unit, but up to now I failed more or less.
I have thought of some possibilities but they don't create the wanted effect:
1. Create multiples of all spells with different manacosts
- it's quite impossible to do that if you want that for more spells
2. Fake the manacosts for all spells
- I wasn't able to immitate that blue shading which shows that you have to less mana
3. Add the saved mana to the unit
- you can't cast the spell when you don't have enough mana for the original costs

So I ask you for either help me with one of my ways or to suggest me an alternative.

Thanks in advance
PS: If it needs to be triggered (what I expect it is), JASS would be desirable.

Multiple levels.

Other than that, expanding on option 2 with some requirements (dummy unit requirement? That would be only one / player though) would be your only option.

You could also make multiple abilities and ad/remove abilities. But multiple levels would be the best.

You could use Engineering Upgrade and reduce its mana cost using that.
I'm not sure if it is reversable though, i think it is just because its an ability, but you never know, i could be wrong.

Thanks guys for your answers. But I think I'll stay with the fake mana and see what I can do to use Griffens suggestion with those requirements since that seems to be the easiest way so far.

@The)TideHunter( thats the way to switch between hero skills and thats the also the problem because you would have to create atleast 2 more for each ability you have

Engineering Upgrade is wicked buggy as is.

Depending on how many levels of it your unit has, and various unknown factors, the ability will stop functioning properly and randomly lose it's toolip and icon...

And then there's the whole thing with it crashing the game if you stick it on a regular unit.

Just stick with triggers, save you a lot of heartache in the future.