When you remove airlines you should have stands or any other means that will prevent car from slamming down on shocks All the way. I used wood blocks of 4x4 with rubber jack stop (to fit into Jack stand locations), so the car is lower than normal Haight but not fully slammed. This way after you put everything together car will be sitting on blocks until compressor pumps up lines and lifts the car off these wooden blocks. If you sit the car up too high the car will be stuck on the blocks after compressor kicks in..not what you want.

So yes, support car, remove air lines, let car rest in low height but on supports, not on shocks (doing so may damage shocks internally).
After you replace repair everything....shoukd be mentioned in the threead, when i had new shock installed, i used compressed airbto gently inflate shock then quickly attach hose and tighten, to put air pressure into lines...helps car be more supported before compressor kicks in.

Hope this answers your questions.

 

Thank You Ciaka. For my GTS, the previous owner had the entire Air Suspension System (including both Air Springs/Shocks, Solenoids Vavles and Compressors/Rely etc.) replaced by an independent shop so I only need to replace the upper/lower control arms.

To make sure I understand this correctly. Even with both airlines off, the system still holds some pressure inside the air tank I assume?

At the moment my car is resting on Quickjacks. When I turned the ignition, the system showed "Chasis System Fault". I think this could be due to the level sensor being at the highest position (car on jack)? To replicate the procedure mentioned by you, could I just lower the jack until the 4 wheels touch the floor to give some load to the suspension (but not fully touch the floor so that there is some sort of support to the body)? Then if I turn on the ignition, the compressor will kick in and inflate both front air shocks in a bit of time?

 

If you remove sir lines. Suspension will droressutize. Your car will slam onto shocks, may damage internals of shocks die to weight.
So always support but at a very low height.
if car low, sendors woll detect and you will have a code. If code is residual, compressor will not turn on until its cleared with a piwis2 etc.
If youre replacing upper arms, youll have to remove shocks which means youll be fisconnecting air lines, etc then remiving shocks to get to upper atm bolts in engine bay.
careful, plastic easy to break. Sensirs, lines, lic. Good luck sir.