This is rather a tool for those who want to transfer their eras from 1.10 to 1.12 and having troubles with their map packs due to illegal terrains like "Ggf"
Once you have any 1.10 version of wesnoth you may get Map Changer add-on and modify your old maps with just couple clicks to be able to run in 1.12 with no problems.
There is a problem that you can not modify your map in 1.12 if it has illegal terrain. Can only be done in 1.10 as far as I know.
If somebody can not find the link to 1.10.7 wesnoth download, please send me a private message and I may help if the link still exists (Despite I believe the link is good, I can not guarantee the link is safe to use, virus-free, etc, so if you ask me for link, keep in mind that it is on your own risk and responsibility. I would prefer NOT to give any links to anybody).
Please post here your ideas, and most important the terrain types that didn't work for you, so I could add them in future versions.
My add-on would replace the terrain 1.12 considers illegal with most appropriate and closest one. You can also customize it for your own liking.
NOTE: if nothing helps, after use of this add-on and saving new map, open it in map editor and go around all edges of map, there may still be illegal map terrain in unplayable areas of map. 1 hex around the map, making a 'frame' of map.. slightly darkened by fog. Change it into what best suits your needs.
(To give you better understanding of what I mean, check these coordinates on any map (top and left sides): 0,0 0,1 0,2 1,0 2,0 3,0 and so on, same on the bottom side of map and on the right hand side of map. There is always 1 extra hex around each map which also has terrain on it, and it may be illegal).
PS. it may be a nice tool for somebody who is making maps also. With 1 click you can change all snow terrain Aa into grassland Gg or whatever other terrain. Then just save map.. Job done. Enjoy!