From #openttdcoop wiki
This page should document the process of game conversion. The plan is to make all games compatible with the latest nightlies of OpenTTD (lets stick to r11615 in this case, which is pretty new). This means compatibility to the next stable release 0.6 as well which will help to provide our games to a wider range of players and interested people.
How To Convert
All games which have been played with revisions below r8979 can be loaded with r8979. This revision is a little bit of a turning point. Don't ask me why, but it works. You can download this revision in the OpenTTD Archive (/the-os-you-like/200703/). Sorry but I don't want to provide the direct links.
Now you need a very special GrfPack (Download) which is a kind of historical rest of ancient times and you have to add the GRFs to the openttd.cfg file, because they are not stored in the savegame.
Of course we have a list of GRF settings for the old games. Load the game, check whether everything is correct and save it again. You don't have to change the paths IIRC.
Now load it with r11615 + GrfPack v6 and check the game and save it again. Add a sign which game it is, i.e. 'Public-Server Game 08' and save the game at a nice entry point like the Network Table or the Message Board, you might add our URL as well. And pause the game.
Now, you are done. =)
Public-Server Conversion Table
|#||Creator||Status||Comment / Message|
|3||Osai||pending||works not with newer revisions (last working tested: r10927). Error: /compile_farm/openttd/nightly/compile_dir/src/vehicle.cpp:225: failed assertion `v->first != NULL'|