There is a KB about it.

IF I recall correctly, the KB said something along the lines of: In newer plesk versions, subdomains were given their own zone file. This caused problems when upgrading/migrating, so functionality was recently added to retain the old way of doing things if need be. You can check which way Plesk is set by looking at a Plesk database table, and you can switch mode in the same way.

Or something along those lines, anyway. I think.