Increase ability level on non-hero units?

Sorry if this was answered already, but I couldn't find it anywhere.

How would I go about making a unit get the next level of an ability through triggers?

Basically, what I want is everytime a certain item is bought the unit selling it gets an increase in attack speed and range. The problem is that I can give the unit the first level of the skill that increases attack speed, but I don't know how to make it level up. Also there can be multiple units of this type, so I can't just store a number for reference.

Just tell it to learn the base ability again.

Also, only upgrades can increase attack range.

This is impossible, you need to use multiple 1 level abilities, a seperate version for each "level". You could store the current level of the ability in the units custom value, and if thats not suitable theres a few other ways too.