First of all, backup of your project, just in case. Just export it as a .drp file.
In the media pool, select everything and hit Ctrl | Cmd-X (Cut). Now that your media pool is empty, you can go to project settings and change the framerate, it will be unlocked. After adjusting the project settings, go back to the media pool and hit Ctrl | Cmd-V (Paste). When asked if you want to change the project framerate, answer ‘No’. That’s it. Now your project has a different framerate.
You can use the same method if you need to change the timeline framerate. Select all clips on the timeline, cut, change timeline settings, and paste back. Just be aware that some of the clips might have a small shift of the in-out points.