Why can't I change frame rate in the export movie dialog anymore?
Changing the frame rate of a timeline at the point that you export a movie can cause problems with edit points in the timeline.
To avoid these issues we removed the ability to change the frame rate on the export movie panel. Instead, when you want to change the frame rate of your timeline, you will have to do it using the Settings panel by tapping on Help and Settings > Settings > Frame rate, then selecting the desired project FPS.
If you have more than 3 clips on the timeline, you will see a pop up panel asking you to either duplicate the project with the new frame rate (recommended) or change the frame rate of the current project.
If you choose Duplicate and Change, then the project is duplicated and the frame rate is changed. You won't necessarily notice this happening but if you look at the name of your project, you will notice that it has been appended with a number and you will also see that the fps has changed. Open the project manager and you can see that your original project is still there.