About the next engines, we shall see. If the T-50-9 has flown, then there only remains one or two more prototypes before the Pre-Serial production commence. They should be able to finnish all the prototypes this year. Its not that Sukhoi cannot recall the first 12 PakFa to factory and refit the necessary equipment. They did it with both Su-34 and Su-35S. And if F-35 is any metric.. There is a 100 F-35 target for block upgrade, even if minor ones.