They've been simply bought. Their leaders have already received several million dollars 'for building a mosque' and I expect they will receive some extra in the near future. Putin gave personal guarantees that their current property will remain 'as is' (which means that everything they had illegally appropriated before will remain theirs de-jure). I'd say this is more than enough to change sides. I doubt they were too patriotic about Ukraine anyway. As for the rest of the population - they all know that salaries and pensions are generally higher in Russia that they had been in Ukraine during Yanukovich's rule and certainly they will be much higher now. Everybody voted by their purse and I don't blame them. I would do the same.