A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
To SDC, the phone agent will uncheck you in after you've decided on the new flight and it's available. You'll probably then need to check in again yourself for the new flight, so don't forget to do this.
If you're trying a redo to attempt to get PreCheck printed on your boarding pass, just reprinting the boarding pass (at a kiosk or ask an agent) seems to work equally well.