Yes, whichever vehicle that is next in the rotation is the "Launch On Need" vehicle for the current mission. When they go to the ISS and they find out that they won't be able to return, they'll have a sufficient amount of time (and oxygen) to get the LON vehicle ready and out to the pad. For the Hubble mission, they wouldn't have enough time (oxygen) to get the rescue vehicle ready and launched. It takes time to get a vehicle ready and rolled out. Michael's comment describes this.