You could do it manually to your gedcom file, it's not too difficult, it's only as text file after all.
All you need to do is edit the record of the person you need to move (and any of their siblings) such that he/she is now the child in the correct different family. For example my record in my ged file has this bit:
1 FAMC @F22@
I'm a child (FAMC) of family F22. If I discovered that my parents were a different couple I could create those people and edit my record accordingly. So I create a new family, say it's F99, and edit my record such that it's:
1 FAMC @F99@
Obviously I'd have to do the same with my siblings but all other relationships would remain correct.
You could create a test tree with three or four generations and try it out, you'll soon see how it works.
