I was able to test this further and I believe we need some slightly adjusted instructions on this. Including those for reference now:
1. Add a domain (in 17) with the same name as the one you're migrating to 17.
2. Stop the SmarterMail system service, IIS site, and application pool.
3. Navigate to the folder for this newly added domain, and remove all files/folders from within it.
4. Paste in the full backup of the original domain from the 15 environment into this now blank folder.
5. Start the SmarterMail system service, then the application pool, and finally the IIS site.
It looks like the presence of the settings.json and accounts.json files from the initial attempt at creating a blank placeholder domain caused SmarterMail not to attempt conversion. When I test this process here it works as expected and the domain comes online smoothly.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com