To update a tileset from 2.3 to 2.3.99 (will be 2.4 in the future) you must add some new graphics and update the file format strings.
The .tilespec file
- Set tilespec format to options = "+Freeciv-2.4-tilespec".
The .spec files
- Set spec format to options = "+Freeciv-2.4-spec".
- The definition of the city images uses a simple, increasing number which is selected via an effect (City_Image). Thus, city.*_0, city.*_8, city.*_12 ... should be changed to city.*_0, city.*_1, city.*_2 ...