Sky can't delete the programs recorded but if the channels are not activate that they were recorded from, then you won't be able to watch them. It's the same principle as recording a movie on Sky Premier then getting rid of the channels, you can't watch the film because you don't have the channels.
Contact them 4-8 weeks before moving and arrange a date for everything to be transferred over then it will all work as before with no interruptions. A close friend of mine was moving and didn't have much notice (RAF), Sky couldn't get out to install everything for seven weeks. The sooner you do it, the likelier you are to get the date you want for installation.