what interested me about this phrase is "Aufhellung," which I'll get to in a moment. here's a quote from MBSI.org:

"Certain ranks in large orchestrions are called foundation or fundamental pipes. These provide a rich bass sound which makes the treble pipes and solo pipes sound richer and fuller. Foundation pipes are never played alone but are always used in combination with other ranks. Solo ranks are those pipes with distinctive voicing which are used to play solo parts (or to carry the basic musical theme) while other ranks play accompaniment."

so, Aufhellung. I think this refers to polishing or burnishing the internal surface of the pipes, which presumably get dusty or dirty after use, and would be done during an overhaul. It may also refer to brightening of the tone color of the pipes, which could indeed be accomplished by polishing, or perhaps by re-shaping the physical profile of the pipe.

i'm not an organ player, but i am an experienced wind-instrument player and work full-time as a flute-maker, and the practice of polishing or re-shaping the inside of a mouthpiece, headjoint, blow-hole or tonehole to give an instrument new life is very common.

i would suggest the following translation, not having seen the rest of the context:

"In the 30's Messiaen also had his pipe-organ overhauled for the first time, the main goal of which was the brightening of the foundation pipes."

hope that helps!