Changing iso in redcine x

Does changing my iso value in redcine x change my middle grey?

I understand shooting higher iso to protect highlights and the opposite for shadows but does changing iso in redcine x after it was shot still have the same effect? I feel like its a dumb question but then why even adjust iso when shooting just adjust it in post?

Yep!

It's worth having an in-camera ISO setting in the general ball park for what you need, since ISO still informs the human operator's visual perception of the overall exposure, so a radically out of line ISO value might prompt other, less desirable adjustments - like stopping down a lens unnecessarily.