I read this and I just had to comment - you've flown 30 years and 1.5 Million miles and this is the WORST travel experience you've ever had? Are you sure? It seems like there MUST be worse. I've flown 2MM across two airlines and this kind of incident wouldn't even make the top 100 list of "worst airline experiences". Granted, most of my bad experiences happened during the 2002-2008 era, but still, "I tried to change my ticket and an agent applied an unfortunately strict but technically accurate version of the rules" isn't exactly on par with something like "I sat on the tarmac for 6 hours with the A/C off and broken toilets" (as used to be more common), or "we almost landed on a house not a runway" (that wasn't Delta).
It seems perhaps you just got a gruff agent on a bad day? Or someone who didn't know the system? Or who was usually a "regular" call center agent but handled your call to reduce wait time - I mean, it may be that you needed to say the phrase "Same Day Confirmed" to this person so that it would occur to them, and then they could process you under that rule.