I mostly meaning distinction between things you can add and things you can't.
Upgrading electronics within original boundaries is totally fine, and they'll do their job(you get radar return on your radar calculated though ew? fire, splinters of modern missile pierce all generations all the same).
Traditional case here is F-15c - retrograde, stable, mechanical flier from 1960s without any fusions ... which still can outbrute most competitors through sheer thrust, power, wing area and so on. Of course, huge AESA, link-16, aim-120d/aim-9x and hmd help.
J-16 is more advanced, of course, but it's still a 1970s airframe at the core. Drawn on paper, for (very ambitious back then!) partial analog fbw on Soviet macroelectronics.
J-10 is a millennial. Yes, millenial born in still poor 1990s China, yes, millennial who went to school not seeing opposite side of the street in horrible smog. But still, she's a modern girl in her 20s. Yes, she is at most 168, not a 6 feet milf.
But,fully digital from the very inception. You can't make flanker into that.