It can't be after WW2 because he'd loose two extra votes in the UN. I can see Transcaucasia being abolished for the same reasons as in otl, but without having the new republics to replace it. Maybe they could be cut up into smal ASSRs or Krais instead. Belarus being abolished on a 'temporary' basis as half the country is under Polish occupation. The one I have a hard time picturing beind disestablished would be the Ukraine. It could start off with border adjustements in favour of Russia and then splitting the administration over what remains of Ukraine to local officials who'd have more power than the SSR government in Kiev and who'd mostly answer directly to Moscow. After a long while this could lead to abolishing Ukraine and RSFSR in favour of a unitary state.