The only caution is that getting those is hard, and made more difficult by the fact that CO looks for KLM availability on a point-to-point basis, while KLM itself allocates inventory using trip logic. For example, if flying EWR-AMS-DXB, a CO agent must see upgrade inventory on separate EWR-AMS and AMS-DXB flights, and will be unable to upgrade you if it's not there even though there could be numerous upgrade seats to be sold on EWR-AMS-DXB as a through trip.