The cheaper fares are due to them having a roundtrip booking requirement (while the one-way fare does not). If you simply cancel the return, you won't have a credit since you no longer meet the conditions under which you bought the fares. If you schedule it far enough out, there could be a schedule change or IRROP's which would still provide value to the return. Or you could change return flight to a different date with no change fee as long as it meets original fare conditions.