Transitionary 3rd->4th generation submarines of Soviet and Russian navies.
Between the project 945A and project 957 there were a series of two subs layed down which could be considered to be 3-4th generation submarines, the project 945AB. Those submarines were to recieve the 4th generation powerplants and new electronics suit, but retain the old 3rd generation hulls.
The new Russian efforts include the upgrades of the Akula (and Sierra) series submarines during their mid life repairs and refits, with new sonar and other electronic equipment. In my opinion it is quite possible that they would receive new conformal bow arrays, as well as new towed arrays.
The current program calls for the upgrade of 6 Akula, 2 Sierra and 4 Oscar series submarines. Oscars would receive different missile launchers (amongst other things), which would turn them into missile trucks with 96 missiles per vessel.