Reuse frames from normal speed animation to save time rendering slow motion Let's assume I rendered an animation at 30 fps. Then I wanted to make it 60fps.
How would I make a script to tell blender to render only the in between frames that haven't been rendered yet? 

I would also need to batch rename files for the image sequence to be correct but I guess that's something else.
Edit: Found "Advanced Rename" which works fine for this last part!
If you rename the already rendered files (using any batch renaming app) like make them all even or odd, then there is not much to do.

Change the speed of the animation using time remapping.



Read:
How can I modify the frame rate without changing overall animation speed?

Then disable the overwrite box in the output settings.



Existing frames will be skipped and only the missing numbers will be rendered.