That already happened when the Soviet Union, that recognized the present and persently violated borders of Ukraine, dissolved and left the Russian Federation as its legal successor. Further, when Ukraine undertook to forego its nukes, that was in express guarantee of Ukrainian sovereignty within the present borders, signed by the Russian Federation.
I, too, wish that Khruschev "with roaches in his head" did not signed over the Crimea to Ukraine, but on the other hand I'd rather have international law respected than another Sov country pop up.
It would be nice if international law were to be respected but it doesn’t always happen. Crimea has been in Russia’s sphere of influence for centuries , and will continue to be for centuries more.