Don't overthink it.
I had trip booked for Sept 30 SYD-ICN-ATL in F and found out mid Sept that I needed to push it a week. Used web site to find new flight on Oct 7, same routing, and called to get it put on hold while I confirmed that I needed to stay another week.
When confirmation came through I went on web site, canceled Sept 30 trip and immediately paid for/booked held trip in same session on site since miles were immediately refunded.
ICN-ATL leg showed as "waitlisted" for whatever reason and then went confirmed overnight.
Process really couldn't have been easier.