Hmmmm... difficult to answer without knowing exactly the purposes of the bag, but here's my two cents. If you've been using a 30 degree bag, you may be able to get away with a 15 or 20 degree. Unfortunately, FF lists only 20 or 10 degree bags on their website, but my experience with their bags is that they rate them very conservatively for temperature. I've always used a 15 degree bag (-9 C) in the Canadian Rockies for early spring to late fall trips, and found it more than adequate. However, a lot of that depends on what kind of tent (if any) you are using, what kind of pad, other people in the tent, etc. Personally, I sleep pretty hot, and I'd find a zero bag WAY too warm for most uses (outside of winter), and therefore it might be a bit limited. Most of the time, my 15 degree bag is open and used like a quilt. However, if you're still wanting to use your 30 degree bag for warmer trips, then a 10 degree bag might be the way to go to give you more versatility.
Bottom line, though, is you're a lucky guy if you pick up a FF bag. I'm jealous!