When a character with cybernetic arms installed levels up, is the new HP amount based off of their original strength skill or the modified skill?

For example, Luke has a base strength skill of 3 and a cybernetic arm for a +2 bonus. His modified strength skill is 3+2=5. When he leveled, would the HP amount be calculated with a base strength of 3 or modified strength 5?

Should be based on the new modified skill and also work retroactively for all character levels.

Are you saying that this is how the mechanics work or are you making a suggestion for how things should work?

It's based on the modified skill.

-someone with 3+2 strength thanks to cybernetic arms

Strength's retroactive anyway, as Lexor said. It doesn't matter when you get strength leveled up - you'll gain all the HP you should have had all along.