New tileset has new tiles the old one doesn't have and since some of the tiles are swapped around you'd also have to edit the old tileset and change it.
And a lot of buildings are made with the new tileset in mind so some buildings might not look that good if it reverted to the old one.
In addition to that, any building interiors that are using custom images that have the tilesets such as doorway images that use the new tileset would look out of place if you changed to the old
Tldr imo you shouldn't have the option to change