If you just want to be able to change return date, it shouldn't be much of an issue and fare differences shouldn't be that big. The cheaper roundtrip fares have minimum stay requirements (generally 7 days) and as long as you continue to meet that when you change, fare differences won't typically be that large (fares for midweek travel tend to be somewhat cheaper than weekend travel). If you wait too close-in to change, you may have issues with finding flights that still have bucket availability in lower classes. This can be particularly true for flights around Christmas when demand is higher. As others have noted, if you are hoping to simply cancel return and get credit, that is a different story as you will no longer meet roundtrip booking requirements of fare and you likely get little or nothing back as a credit (unless you use it for a return flight from US to EU to meet original roundtrip booking requirement of fares).