Commissioning of Existing Equipment

This would be a very good question for GBCI reviewers. There is no defined (that I know of) guidance for this kind of situation. For sure you should Cx all systems that you are touching or extending. For example, if the whole space is served by four air handling units, but only two serve the area of the renovation, then the two serving your area should be commissioned. But for the rest, it seems that some recognition of previous work is appropriate. As with any time I recommend talking to reviewers, have the answer you think is reasonable ready. Go in saying "we do not see how the prereq or credit directly addresses this situation, but we have reviewed the goals and basis of the standard and feel the scope of Cx should be..." Have a clear justification for your plan based on the intent of the prereq and standard.