Before upgrading, I made a full copy of our forum and DB. I then performed the upgrade on the copied forum, and ran into the Style challenge mentioned above. I confirmed that the CSS and TPL files in that upgraded forum folder were copied correctly, and did not make any changes after the upgrade, but the old forum styles persisted.

The key to all this is actually the Admin / Forum Settings / Forum Address (URL) option. When I originally copied the forum, I did not make any adjustments within Settings, so Forum Address was still pointing to the (original) production forum, which had our existing style changes. And, as I found out, Style information is pulled from the URL specified in Forum Address, and does not use a relative path to the current forum folder. You're likely aware of all this, it just happened to be new info to me.

Joe