Can I exchange a unit for a different course?
It depends. Units are specific to a course - a ProFirstAid course unit can only be used for a ProFirstAid certification, a ProBloodborne course unit for a ProBloodborne certification, and so on.
However, you may be able to exchange a ProFirstAid course unit for a ProCPR Basic course unit, because they are both layrescuer courses and they cost the same. Exchanging units between course types - like a ProFirstAid course unit for a ProCPR course unit, or a ProFirstAid course unit for a ProBloodborne one - is more difficult, because they are different types of courses and have different prices.
In both cases, your Group Specialist is the best person to help you navigate the choice to switch to a different course, and can tell you if your units will transfer.