Typically players only make the switch at the end of their contract these days, so as far as I'm aware, there's not much friction around it. Although I think Sonny Bill Williams caused a mess by switching to Union about 18 months into a 5-year contract, and Toulon ended up paying a transfer fee which was essentially buying out his contract (similar to Neymar to PSG in a way), but that was over 10 years ago. He was banned from the NRL as well.
Bear in mind though, it used to be that most players switched from Union to League, as League was professional and Union wasn't, so it was a way for the best players to get paid and they didn't have to break any contracts to do so.