Guys, he's wanting to tune up 2 steps to Ab, not down 3 1/2 steps to Ab.
He'd need a lighter string gauge. Heavier strings wouldn't be able to take the strain of the tension necessary to tune up to Ab. He'd need lighter strings in order to be able to tune up that high. Plus the increased tension would bow the neck something fierce! it'd take a pretty major truss rod adjustment to get it playable again. And the slightest barb in the nut or the saddles would cause strings to break left and right. Not to mention the wear and tear on his fingers from the higher action and stiffer tension.
To the OP:
Why not try one of these 2 things instead:
- buy a capo and put it on the 4th fret. It would require no new strings, no truss rod adjustments, and barely even any tuning adjustments.
- If you're dead set on tuning upwards to Ab and don't want to use a capo like most people would, I'd suggest using the same gauge strings, but move them all up a string. Meaning, use your A string as your 6th string, use your D string as your 5th string, G string as your 4th string, B as your 3rd, E as your 2nd, and find something like a .008 to use as your 1st string.