Is it safe to replace a 6/3 soow on a 6-50 adapter with a 14-50 adapter? I have an additional 6/3 soow to use. The adapter says it must be used with a 4 wire, but the charger only uses a 3 wire cord currently. Do I leave the ground disconnected and just use the hot wires + neutral?
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.