You can replace the receptacles with ones meant for 240VAC at the same time you modify the wiring. Then it should be okay. Replacing the NEMA 5-15 receptacles w

You can replace the receptacles with ones meant for 240VAC at the same time you modify the wiring. Then it should be okay. Replacing the NEMA 5-15 receptacles with NEMA 6-15 receptacles would work.