I can't imagine there being any trouble, provided that you retain BTM-2015 until the tournament is completed, AND that you back up your 2015 tournament (which should be a standard practice anyway) before doing the upgrade. Note that once the tournament is opened in BTM-2017, it will not be possible to open the same tournament file in BTM-2015; however, the initial installation of 2017 should copy all existing tournaments into a separate Tournaments folder for BTM-2017, keeping your 2015 work available.
If you need 2017 and/or PRO items at this time, then by all means (such as, introducing Brackets to future squads). If, say, you're using both BTM-2015 for the tournament proper and TBrac-2006 for Brackets, a PRO version of BTM will alleviate the multiple entry of names and scores as they would be integrated.
Always check, download, and install the most recent update of your software to ensure that issues may have been corrected.