Or am I missing something really obvious?

(Using BIAB 2006 BTW)
I have a song that begins in C, then has a section in Ab and another in E, before returning back to C. I put all the chords in already. Now I'm trying to put the melody in using the notation window. At a certain bar I want the key signature to change in the notation window. I don't need any of the chords transposed. I think I read somewhere on a FAQ that to change keys you would use edit-transpose. But that's not changing keys, that's transposing. I also searched the forums but I only saw mention of this transposing method. Using this I would have to re-input the chords (not that big of a deal) and I still don't get a displayed key signature in the new key (what I'm really after here). I'd like to be able to keep inputting the melody without worrying about accidentals for every note.

I can't possible believe that a program that has so many options, that has covered so many possibilities, would not have the ability to do something sop basic. But if it's doable, it's certainly not obvious. Please tell me what I'm missing.

Thanks.