Say I want to mostly animate on 12s, but I also want to have the option to put things on 24s if I so choose.

What would be easiest is if I could simply set the file to 12s, so I could animate easily on every frame. What I'm wondering though, is if once I have an animation on a 12fps project, if I can "convert" the file to 24, so that the frames are automatically spaced apart, now having an empty(or held) frame between each one? That way I could have the convenience of animating on 12s in a 12fps environment, then go back in and add more detail in places if I see fit by adding in twice the frames.

If not, I would have to simply skip a frame every frame on a 24fps project.

Thanks!