You will need to make sure your app is as optimized as it can be if you were to receive 2k visitors per hour. With Bubble’s new WU based pricing plan, you get charged for each visitor (page load).
You’d better ask support team.
Make some exercise to estimate WU consumption over an average visit and extrapolate to your load, compare it against bubble pricing.
Migration should actually be planned from the beginning… Now if both app versions have common database or your app is stateless (i.e. no user data is stored), you can consider load balancer/reverse proxy scheme to gradually switch between apps. But it’s a rare case it is possible if you didn’t plan it from the beginning.