altering a jacket

[Jas0n]

To date, I've only had to take in my jackets. How much do you think I might be able to let out the waist on a jacket without altering the sillouette? The shoulders are perfect and so too is the chest, but the waist on this particular jacket would need another 1" or so let out. On a seperate and purely hypothetical note: can a single vent jacket be converting to dual vent?