Originally Posted by
PTahCha
Fare basis is not relevant to SDC. If a seat is available and it's not oversold, then it's yours.
This has not been true for quite some time. Fare class was not relevant for the PMUA standby policy, which was more restrictive for times - you could only request the change after checkin eligible and to a flight within 3 hours of the request.
The policy currently in use is the PMCO one, which allows SDC at checkin eligibility and to any flight within 24 hours of the request. However, if the original fare class is not available, you must also buy up to that. IMO, also options won't show up on the kiosk/website/app if the same fare class is not available, you have to call in to take advantage of that.