I did a similar work to my friends 8V 2.0 TS, and I did it because the car wasn't heating up to normal operating temperature.
Changing of the thermostat was easy, however getting the air out was not. After few attempts, I consulted the owners manual
. In Finnish version it said "Do not drain the system, filling requires Alfa Rmoe trained tecnician. " English versions seems to say "To get this operations carried out, contact exclusively Alfa Romeo technical service."
In real life it means that if you want to get the air out, you will have to fully understand the system. The you can start the hose job, which I did.
Get some extra 19mm (? if I remember correctly) hose and attach it to the thermostat (top outlet). Also add hose to the heaters hoses make sure that there is no upsidedown U-loop left in the system that could catch the air. Fill with water through these hoses, and finally when you've filled everything - quickly put the the original hoses back where they belong and make sure air doesn't get in.
I hope this brings any clarification, but the main point is: The cooling system does not circulate the air out itself. If you fail to get the air out you may blow your gasket or crack your head. frown