I am in the process of fitting Eibach lowering springs to my 147 next week by my Alfa dealer.
I spoke at length to my Alfa Service Manager about this. Basically I've been told that if you don't change the dampers, you will halve the limetime of them. If you can imagine, with the lower springs causing a lower ride height, the dampers will be a half compression all the time. This will not affect their damping characteristics, but affect durability.
The Service Manager brought out a set of adjustable Koni's that were due to be fitted to a 156. They were quite impresive, and he told me that since they were going to hoist my car up to fit the springs anyway, he would fit the dampers at cost price (ie: no install charge). I thought about it, but told him that I was extremely happy with the damping rate of the current/standard units, and would consider fitting new dampers, when the original one's gave up. Still I can change my mind before the fitting.
Finally I was told that only the front dampers will need to be replaced....Rear dampers can be left alone as they don't deal with as much weight as the front dampers.
I hope this helps, and if any one has info that can clarify this, please reply.