When we say use Su35 some people say it is new plane but in this case how can commonality come when S35 is different from Su30mki and same way as Mig35 to Mig29
the new plane dose not mean its totally new,
it means many components or criticle systems are new, the basic design may remain same,
many a times the new system evolves from the older systems and share many things with the old system,
that makes the old infrastructure useful as with the little change in the exsisting infra you can use it for the newer systems,