I gave my family member a mileage upgrade to Delta One for the outbound and the return on itinerary this coming fall. Both Delta One segments upgraded immediately upon request. However, two different agents have told me that mileage upgrades only upgrade the Delta One portion of the itinerary and that the family member will either have to pay to upgrade the domestic comfort plus segments, or have me use a second Global Upgrade Certificate. Are mileage upgrades to Delta One only for the Delta One segment? Both agents were firm, pay to upgrade the fares to domestic F or burn a GUC. I offered to burn a regional certificate and was told no way, either burn another GUC or pay for the additional fare for the domestic segments. if this is the case, the mileage upgrade benefit is not what I had imagined.