If I uninstall the Shop mod, does it also remove the database entries? Which means if I reinstall the Shop mod it won't recognize the existing credits that the members have already earned?
Yeah, it removes the database entries
. What you could do is make a database backup before uninstalling it. Then, uninstall it, and straight after this, restore the backup. Then, when you install SMFShop, it will detect that the database is already configured, and won't do anything.
NOTE: I would suggest that if you do this, put your forum into Maintenance Mode so you don't lose any posts.