I’d disagree with this. It’s still Type A even when converted, as it primarily refers to the size of the postbox (the main body), and if you add the “_parcel” suffix to it, a search for “post_box:design=type_a” could omit the parcel postboxes in the results.
Aren’t parcel_mail_in=yes and electricity=solar sufficient tags to differentiate these converted postboxes from others?