If you want examples, just look at J-10B. Fundamentally it's the same plane as J-10A, even the engines are the same. The only upgrades are the air intakes and avionics. J-10A matured for a decade already and yet J-10B still hasn't officially entered service. If it was so easy, we would have 20th generation fighter jet by now, instead of the current 4th gen J-20 and F22.
Off topic:
First batch of J-10B has already entered service. They just didn't make a big deal out of it yet.