How can I revert from dynamic stretched 16:9 aspect ratio to 4:3 aspect ratio?

Hello Everyone,

I've got an aspect ratio issue that I was hoping someone might be able to help me with. I have a video that was originally shot in 4:3 but was later converted to 16:9. The aspect ratio wasn’t simply changed by stretching it to 16:9, rather the sides were stretched so that it would fit the 16:9 ratio meanwhile keeping center of the frame unaltered. Basically, I can’t fix it by changing the aspect ratio back to 4:3 because doing so will compress the middle but still keep the weird morphing, stretching effects on the sides.

From all the research I’ve done online, this effect seems to go by a few names: Non-Linear Stretch (NLS), Dynamic Stretching (according to GoPro), and Atomic Stretch (by Atom Image Labs). I tried a plugin called Andy’s Elastic Aspect on FxFactory, in addition to Atomic Stretch but again, neither of these plugins reverted the stretch back to 4:3. The only one that I found that was close was the GoPro FX Reframe plugin. By changing the projection to “HD 4:3” and checking the “Undo Superview” checkbox I was able to create a pretty close copy of what the original would have looked like but there is still some morphing on the edges.

Does anyone know of any plugins or techniques that I can use to fix this problem?