Technically, the agents are supposed to follow the routing rules for your fare. Looking at currently published fares for DTW-LAX, they all have a DL501 routing code which requires non-stop routing, so you got lucky with that change. Most of us book separate mileage runs rather than trying to endlessly game DL's same-day confirm policies. If you take this to extreme, I'd expect agents to start making notations on your tickets at some point to stop granting you favors. Ultimately, if too many people abuse the system, I'd fully expect DL to start cracking down further on the same-day change rules.