I had this very same issue - at the end of a large grading session, I discovered my timeline was set to 24fps instead of 25fps. All the posts I found said "It's not possible to change the settings", but in the end, I managed to do that!
You need to go to your Resolve projects directory (here it's C:\Program Data\Blackmagic Design\Support\Resolve Disk Database\Resolve Projects\), and open the project directory.You'll see a bunch of folders and xml files. Now, you need to edit the XML files and search for some strings and replace these with some other values:
Changes I did:
project.xml:<FPS>24</FPS> ===> <FPS>25</FPS>
<FrameRate>00000000000038400000000000000000</FrameRate> ===> <FrameRate>00000000000039400000000000000000</FrameRate>
config2.xml<FPS>24</FPS> ===> <FPS>25</FPS>
<FrameTransmitRate>24</FrameTransmitRate> ===> <FrameTransmitRate>25</FrameTransmitRate>
sessions/some-code.xml<FrameRate>00000000000038400000000000000000</FrameRate> ===> <FrameRate>00000000000039400000000000000000</FrameRate>
That's it! I have my project set to 25fps and I'm delivering at this frame rate..
Hope this helps - I don't see a reason why this cannot be changed in Resolve directly....