Replacing Shut Off Valve
As long as you can get the existing valve fully open you can leave it in place.  Buy a piggy back valve, you screw it into the existing valve and use it instead of the original.  

As zelinka mentioned the existing valve is done in expansion pex and the tools needed to put a new one on are close to $600.  

You could probably do a sharkbite fitting on the pipe if you cut it below the expansion ring.  Something like this:

https://www.homedepot.com/p/SharkBite-1-2-in-Push-to-Connect-x-1-4-in-O-D-Compression-Chrome-Plated-Brass-Quarter-Turn-Straight-Stop-Valve-23337-0000LF/202721912