Well, the UNSC and the UN in general would come down hard against such territorial aggrandizement. The NATO/OTAN would seriously discourage any member states to do this, ditto for the EU. Harsh economic sanctions would be passed. It's true that the same applies to present-day Russia wrt Ukrainan territory, but keep in mind that the countries you mention as possible expansionists aren't nuclear powers and weren't as strong, militarily, as Russia is.
If these forms of pressure don't work, Russia would probably be happy to fight to defend Kaliningrad or Karelia; nothing helps internal trouble as much as a external enemy.
Ukraine would probably fight and at the time is still a nuclear power, too.
Moldova is too small to fight a Romanian invasion on its own, and it probably wouldn't even want to do so - but there's the small problem of Transnistria and its Russians. It is a problem to date, and it would elicit the same reaction as for Kaliningrad.
Yugoslavia is another can of worms, and it is possible that in the general chaos of the local civil wars, a partial annexation by one of the neighbors would be militarily successful. The country doing that would, at first, possibly even be excused as a bringer of peace and stability. But over the years, it would become a pariah state internationally; and, locally, there's the usual Balkan responses to this: guerrilla and terrorism. And to gain what? An underdeveloped border region that costs more than it yields. The gain wouldn't be worth the effort, and indeed, it's not coincidental that nothing of the sort happened.