One of the hardest transitions I ever did was inverted to non-inverted mouse. Like others said, I started on it early. my FPS experience started with Doom 2 and only using the keyboard (since you couldn't look up and down anyway). Then eventually into games which DID have mlook, like Duke3d/Shadow warrior, but it wasn't until Quake1 with KB only that I got convinced to try mouse, but used inverted. It wasn't until pubg where I made the change to non-inverted when certain times fighting recoil inverted was just too difficult. It got me in a LOT of trouble for a while though, kept trying to control recoil and aimed even higher. Woops.
Now i'm used to it and it just feels natural.