Rendering every second frame to increase overall fps

Hi, I made a 4 seconds animation at 30 fps. That means I rendered 120 frames.
Only after stitching frames into video I realized I need the animation to be slower.
So I want to stretch it to 8 sec and keep it 30 fps.
Is there a way to tell blender to render only frames in between the frames I already have, so every second frame?