Hi Steve
I have faced this issue a few times, and the most workable method I have devised is to use the existing budget buckets for the forecast values. Then create a new set of acct/depts making the acct/dept identical to the real acct/dept but put "B" before the acct code (e.g. B4000 instead of 4000). Make these "B" accounts inactive. Import the original budget figures here.
The advantage of doing this is that you only have to fool around with the B accounts once per year. Also, you can easily design financial statements to strip off the B prefix, showing the original and forecast figures in adjoining columns on a financial statement. If you need help doing this, I could send you an example.
Steve Schwartz