That's a classic series story. Nothing to do with the new show
It's also the only episode of the classic series that was done on film,
Technically speaking there are 5 other episodes of Classic Dr Who from the 60's which could get an HD release. For example one episode of Dalek Invasion of Earth was shot on 35mm as were 4 other episodes. Terror of the Zygons coud get an HD release as 3/4 of it was shot on 16mm film and most of the film segments still exist.