OK, I didn't realize LX was trying to emulate Ryanair on flights to GVA.

So I can either pay €30 each way or buy the Flex fares directly from swiss.com and refund the ticket(s) purchased through united.com.

The difference between the Light and Flex, depending on the flight, is less than €30.

I can't find anywhere on swiss.com how much they charge for the checked baggage so I'll have to hope the ticketing email is correct that it's only €30.

I've read accounts of people having to upgrade from Light to Flex fare by paying the difference between the fares but then having to re-queue to check the bag.