Have done it during delays or when the website pricing is odd or to meet a deadline. I call when I have the chance and the agent cleans it up. But trying to game the upgrade availability or to maintain flexibility by holding two less-flexible fares? This type of thinking is what propelled international business award and upgrade availability into the toilet, as over time Delta closed the loopholes. Also the ever-tightening rules for higher status, the 72 hour no-cancel rule for award travel, etc.