Just because the last segment is in different class does not mean the fare is not fully refundable. That has nothing to do with refundability. As long as the fare says "Cancellations Permitted" in the Penalties section of fare rules and there are no cancel fees listed in Penalties section, the fares are fully refundable with no fee for refund no matter what booking class you are getting on last hop. Just did a spot check on ex-US to EU refundable fares on EF (fare basis code will end in a 9) and none had a fee for refund. When you buy a roundtrip to EU from US, you are getting ex-US fare filings in both directions. Fully flexible fares are just refundable fares with no advance purchase requirements. There's no need to upfare to these if you just want the option to fully refund before flight.