[ArtieBob]

IMHO, some of the confusion may relate to two items. First, the length of time a major component of an aircraft, i.e., the fuselage may have been in inventory before it reached final assembly. I am not as familiar with the details of Bf 109 manufacturing as with some other Luftwaffe types but generally, fuselage, wings, power “egg”, empennage, etc., might not have been manufactured at the final assembly point. The fuselage generally carried the RLM W.Nr. for the aircraft and, in cases that can be documented, the RLM W.Nr. was assigned at the fuselage subassembly point. There might be weeks or even months between leaving the subassembly site and reaching final assembly, photos show fields full of Bf 109 fuselages awaiting wing, power eggs, etc.
Second, it would appear that whether an aircraft was considered Neubau or Umbau would depend when the BAL acceptance was made, thus a fuselage intended to become a G-6, if it was in production inventory, might be modified before final assembly and BAL acceptance to become another subtype and still be considered Neubau. If one looks at the acceptance sequences they are generally not in W.Nr. sequence. A “new” aircraft might be flown directly to another site following BAL acceptance and modified would be considered Umbau. Of course, when dealing with almost any aspect of Luftwaffe production during the last year of the war, exceptions are the rule and documentation may not exist. The goal was to push completed aircraft out the door (cave, or from under the trees).
Finally, IIRC, von Lutz may have firsthand sighting experience on this subject.
Best Regards,
Artie Bob