Originally Posted by MikeBOS

The answer here is actually quite clear, but finding it out, in my experience, requires that you bump up against the rule (as I did). You can do what you did as long as you have the same fare code for both legs of your connection. If you have different fare codes for each leg (i.e. it's ticketed as two back-to-back one-way tickets), no amount of pleading, logic, EXP desk calls, anything will get you on the alternative flights. In my case I was looking to stand by on a mostly empty non-stop, vs. 2 packed connecting flights, and they absolutely refused to let me on the non-stop. I tried the EXP desk, Admirals Club, gate, asked nicely in all cases, and had absolutely no luck.