I am not sure Russia (and Belarus, who would view that as a very dangerous and threatening move and may even be the ones who will petition Russia to launch a joint operation to intervene to stop it) would allow that, but if they do then it will be because they have decided to do away with Ukraine altogether. And that may trigger some of the more fanatical Ukrainian nationalists to decide that they want to fight the Poles too if they perceive Poland to be moving toward annexation of the Banderite heartland. For all these reasons and more this is much too messy a proposition even for the insane Poles, so i put the odds of that scenario fairly low.