I have serious doubts the story of the couple moving into the stable block is true.
The stables have been converted into small two bedroom flats that have been used by staff and rented out to public people as well. I have found pictures of one of them that belongs to an architect.
Though they may be downsizing, the couple is not likely to move into a two bedroom flat. They use their apartments as their office as well. There are other larger apartments that are free in the palace including the one that belonged to Christopher Geidt which was considered for the Sussexes by many. Its definitely smaller then apartment 1, but would still allow the couple offices and room for their family to visit.