By default, 1factory numbers balloons as integers (1,2,3,...). however, you can create a balloon with a decimal (e.g. 2.1, 2.2 etc). This is useful when inserting a balloon after a drawing rev change.
First add a new balloon "Shift + Left Mouse Click". Let's say you added balloon #10. Then on the slide-out that appears, change the balloon number to 2.1. Fill-out the rest of the information and click "Save".