The only thing that the T-72 and T-90 have in common is a similar chassis and a similar canon. Yes the T-90 is a further development of the T-72; however, the T-90 has evolved so much that it can not be considered a T-72. The T-90 has improved armor, different turret, a better and longer range gun, improved autoloader, improved engine, ect. T-72 can have a lot of the upgrades from a T-90 including a new turret but then it is only a T-72 in name.
Older T-72's are outdated unless they are upgraded. If we speak of the T-90MS that tank is even more radical and different.