This wiki was created to distill SDC (specifically not standby) down to a set of rules in the hopes of benefiting others.

The official "rules" can be found here. In practice, some of the rules almost always apply almost, and some are more flexible.

Note there have been reports of SDC to one segment removing confirmed CPU upgrades on downline segments. If maintaining the confirmed upgrade is important then these changes should be handled with care.

Most Frequently Asked Question: When do I call/go online to make the change?
Follow this flowchart:
Question 1: Are you within 24 hours of your first originally scheduled UA(*) operated flight that you're trying to change off of? If yes, continue to question 2. If no, continue to answer 1.
Question 2: Are you within 24 hours of the first UA operated flight you're trying to change on to? If yes, continue to to answer 2. If no, continue to answer 1.
Answer 1: No, you cannot call/go online yet to make an SDC.
Answer 2: If the answer to both questions is yes, then you can call/go online to make an SDC.

General Baseline Rules

These are the general rules that usually apply to SDC's; falling outside these rules usually requires payment of a full change fee and the fare difference:
How to SDC

SDC can be accomplished in a few different places (in rough order of likelihood of being able to get the rules bent):
NB: An SDC where the original itinerary (in one direction) has three or more segments cannot be done online.


Nuances and Loose Rules

Now for the hard part: figuring out where the flexibility is in the rules. This list is compiled from the responses to people saying "Can I do X?" where X is something that is not clearly permitted by the rules. One's likelihood of success here depends a bit on the method used for the SDC.

For each item, the following codes apply:
Y = possible using any method
A = possible using an agent (phone/counter), perhaps having to HUACA
A* = mixed reports of success with an agent (phone/counter),
N* = possible only if you find a very accommodating agent
N = little or no reported success.

If you encounter issues with an agent (phone or counter/gate), you can ask the agent to reference GG SDC. Additionally, GG SDC line 61 specifically references that fare classes are supposed to level out 3.5 hours before departure, so you might be able to use that to your advantage if fare classes have not leveled out and you're trying to ask an agent to open up the correct inventory or force you onto the flight.