When I change video profile the project gets corrupted

Hi,

I recently built a ~40 minute-long project with many photo/video/audio clips and text overlays and chose a 720p @30fps profile. It renders fine in that video profile (though strangely, for some reason finishes and always says it crashes at the end.. but the resulting output file plays fine all the way through..?) but I have need to change this profile to a lower FPS to try to allow it to play satisfactorily on some older equipment.

When I change the profile from 720p 30fps to eg 720p 25fps (I've tried several other profiles) half the time it locks up and doesn't come back (have to kill it). The other half the time it completes and the project timeline looks OK but I get errors if I try to edit something (unable to move clip, etc etc). If if I save it, quit and go back into the project I get a warning while loading about overlapping transitions and everything in the project is offset and messed up. So, that project file is now junk and I have to go back and try again.

I'm not claiming to be any kind of expert here, so maybe I'm doing something wrong. Is it proper to want to change the video profile inside the project and expect it will not corrupt the project timeline?

Thanks...