OP here. thanks for all the replies. I chuckled when I read them b/c I'm now more confused then ever. (Kinda reminds me of moment in college when I asked a panel of three "science-types"/PhD's a SIMPLE question and got a PhD panel answer. I was so confused....

Alas, I'm more confused. So to reiterate, I've now checked and I'm on an A fare and an L fare. If I want to SDC and I'm ok keeping the routing the same, which fare rules will apply - the A fare class which means only a FC seat needs to be avail or the L fare class which means there needs to be specifically an L fare on the new flight I want? is the answer that the fare class will apply for each respective segment for which I'm attempting to SDC (i THINK that's what people are inferring?)

Also, finally, I'm not sure it matters at all but some have mentioned GAP fares on delta.com but I called the res agent when I bought up to FC so it was not (re)issued over the web, it was done w/ a res agent. Thanks for any further clarity.