I was more concerned by the upgrade of jail directly via pkgbase itself rather than the conversion since I didn't see(may be i missed it) any procedure or notes about it.
Frankly I didn't even think what you did was possible but now that you tested it I am actually glad to know that it's working this way too, this will prevent people to rebuild all their old jails from zero.
Well done mate thank you for the report and the demonstration.