1. The seat guarantee works automatically, by the following criteria:

-There is an AC*SE in the PNR
-The booking class of the segment concerned is in Y
-The segment is in HL status (so if the waitlist is closed, even the guarantee can't help you)

1. So if you change dates, your new segment will be either in HL status or waitlist closed. If you get HL, then when the agent ends all transactions and re-opens the file, the HL will be changed to HK and a note added by the computer to your file that the seat was granted through the SE booking guarantee.

2. If you manage to change dates this way, there's nothing stopping you from waitlisting for an upgrade on the new date.

Not many people use it though, because of the Y booking class requirement. Not like CX where they guarantee most discount classes; much more useful! (And available to middle-tier too).