I think it was treaty between countries instead of cabinets. Even if the cabinet changes, that should not affect the treaty in any way. And even if there wouldn't be that treaty, annexing parts of other country like that, is illegal.
Almost anything that you heard from Putin or his followers is a outright lie, bad excuse or distortion. Like the lie that it is not his soldiers operating in unmarked uniforms in Crimea or that they have reduced the the number of soldiers on border, waiting to attack Ukraine.
And there was very little threat against Russians speakers in Ukraine. Protecting them was just another excuse. And now Putin is on purpose causing those troubles in eastern parts of Ukraine. Russia is instruction Russian citizens how to go eastern Ukraine an without causing suspicions and then how to get a weapon there, uniform etc.