Converting EVSE with 6-50 to 14-50
If your EV charger has worked so far with a 6-50, or is shipped with a 6-50 adapter, then it probably doesn't need neutral to function.  That's no surprise, honestly.  

There is nothing wrong with changing the plug to 14-50.  In that case, you simply do not connect neutral in the plug.   

You certainly do connect safety ground!  In all cases!  

In cordage such as SOOW type, the /3 designator means it has 3 wires: black white and ground.  (/2 is groundless, /4 includes a red wire).  

Use the white wire as one of the "hot" wires.  The usage will be obvious in this case, but bonus points if you color the wire with a sharpie to indicate it is not neutral.