4 Dogs and Conditional Probability of being selected

4 dogs-Rover, Pepe, Spot, and Scruffy- are in the pound wondering when they will be selected; Christmas was coming and it was an exciting time based on previous years.  Rover, one of the dogs, heard a rumor that one of them will be selected before Christmas at random and felt his odds were 1/4 of being selected.  However, due to a mistake in paperwork, the dogs with names from "S" were not included in the list to be selected, so they were left off the list.  Rover overheard this mistake and decided that the probability of being selected were just improved to 1/2.....is Rover correct in thinking that his probability changed from 1/4 to 1/2?