Originally Posted by
hhdl
It's interesting to contrast those with, e.g., DL. Fee is a uniform $75, waived for Gold (Platinum equivalent) and up, as well as on refundable fares. Main Cabin fares require availability in the same Main Cabin fare class; paid Comfort+ (MCE equivalent) gets last seat availability in Main/C+. PE/F/J fares get last seat availability in booked cabin or lower. Paid or complimentary upgrades do not transfer (this acts as a subtle disincentive for elites to SDC, except perhaps to later flights). Can change connecting points, but can't change connecting to nonstop (I forget if the reverse is allowed). Coterminals (e.g. JFK/LGA/EWR, DCA/IAD/BWI, ORD/MDW, etc.) aren't allowed by rule, but the rule is customarily ignored for Diamonds.
AA upgrades transferring is a substantially more liberal rule than DL's, but special inventory would be substantially more restrictive.
As noted, DL requires original booking class inventory unless you have a paid C+ or FC fare. In practice, this significantly limits SDC availability when on cheaper Main fares. Also, many DL fares on non-stop routes have a non-stop routing requirement in fare routing rules. Agents seem to be able override routing rules in fares, but I've also had an agent refuse an SDC request from non-stop to connecting when on a fare with non-stop routing requirement. DL online SDC enforces routing rules in fares. Although if there are flight delays (I believe threshold is 15 minutes or more), it will open up numerous alternative routings for no fee changes and waives original class inventory requirement.