I can find no information about problems with unmatched transactions and upgrading. However, if it were me, I would match before doing so, just in case. Backing up is always a good idea, yet, if a problem arises and you have to restore, it may take more time than the amount you intended to save in the first place. Make sense?
Also, I cannot find the system requirements for 2015 as it appears to be discontinued. If memory serves, it is unsupported in Sierra, but still may launch. Since you have 2 systems, are you able to process the full upgrade on one and refer to the other if there is a problem?