I dont how OP you never knew the SBY policy that CO put in place some time ago

Sorry but in your case, thats exactly why CO has made a Fee involved! all too often people purchased the cheapest fare figuring all they needed to do was show up at the airport and fly out SBY on an earlier flight and that way get around having to pay the expensive fare. I dont blame CO for this reason.(Im NOT saying that was what you had intended along, but please keep in mind its what alot of people do or better said did have as their intent when they purchased their tkt)

what CO is suppose to be no longer charging at some time would be in your case the IAH-XYZ. In other words had you flown the DAL-IAH and then either got in before you were suppose and thus can now catch an earlier flight from IAH-XYZ or the earlier flight that should have left before you landed for some reason was delayed and now you can catch it, for this CO wont charge for the SBY, as its impossible for anyone to Game CO since theres no way to know that on such and such future date that Flight 3121 will ariive early or that Flight #212 will be delayed. Keep in mind this will only be allowed if No bags have been checked, if the passenger checked any bag/s then no SBY

Personally Id like to see them add SBY for free,day of- for Award tkts, If I reserve a red-eye say LAX-EWR since thats all thats available and the day before I see all the flights have availability I dont know why CO cant let me do SBY for $0. There was no Gaming going on Fare wise. ill take this up once they allow free SBYing for connecting flights