How can I replace a NEMA 6-30 cord & plug with a NEMA 14-30 cord & plug?
NEMA 14-30 is 120/240V, with neutral to provide 120V from either hot to neutral. Hots are normally black and red and neutral is white.

NEMA 6-30 is 240V only with no neutral. Presumably in the old cord, black and white were the hots.

You need to cap off the new white wire (neutral) and connect the red (hot2) in its place to L2.