Make some hosts on version 1.8 and export them to XML.
In trunk version make same hosts. Make sure the name is the same, but add different parameter for default interface, like different IP address.
Now import XML to trunk. If you check only update or create new hosts, it works but it does nothing, it does not override the default interface. If you check update or create new items, import fails because in trunk we already have a default interface and it is considered a different one. We stop with error: "Host cannot have more than one default interface of the same type."

Should we then a) create a secondary interface with same type which is not default or b) since we update host data, override the default interface with the one in XML ?

Subject open for discussion.