You're assuming the people manning your 1940 base are trained to 1940 standards. If they can import F35 parts they'd be downright stupid not to import people trained to 2006 standards.Hardly.o: There is a technology skill level passing over 3 TLs much less one. Perhaps TL12 can do basic module replacement for TL13, etc.
Class A starports on TL 5 worlds can perform annual maintenance on TL15 ships. I should hope that naval bases on TL12 worlds would be able to maintain and repair TL15 ships. That's what they get their pay for.
Hans