I appreciate that IMatch 2017 install allows me to continue to use IMatch5, because I'll need time to migrate some scripts I've developed that are important to me. When I opened IMatch2017 for the first time, it found the current IMatch5 database and offered to update it warning that the resultant database would not be usable by earlier versions. So, I obviously have not done that at this time. But is there some way where I can create an IMatch2017 sandbox based on the current state of the database in IMatch5, which I can then ultimately convert from the active database I use now?
I'm not quite sure that I can follow your question.
When IMatch 2017 opens a database, it must convert it to the new format. IMatch 2017 introduces many new features and these require structural changes in the database. Older IMatch versions cannot longer open the database.
You should make a copy of your database before opening it in IMatch 2017. Then you can use the original version with IMatch 5 and the new version with IMatch 2017.
See
https://www.photools.com/imatch-2017-whats-new/ (Under the Database System Update headline)
https://www.photools.com/5515/uninstalling-imatch-2015-installing-imatch-2017/
You don't need a "sandbox". Just make a copy of your IMatch 5 database (it is only one file) and use the copy to test IMatch 2017.