Once you have begun using an observation set, you should use caution if you choose to modify it. For example, if you add items to a list, the reporting feature for that list may not provide accurate information for observations performed prior to the addition(s). However, if you change the spelling or wording of an item, reporting might remain accurate, depending on exactly what you change.
A better option might be to duplicate the observation set, modify the duplicate, then begin using it for subsequent observations.