This is not exactly a bug, but it's a major flaw in the game - you can only do a promotion once, and if you screw it up, that's it as far as the game itself goes. However you can hex edit your savegame to get over this. You should be able to find the details in past posts on this board - a good starting point would be to look for Klar's posts.
But be warned - there are very very few ways of screwing up your files to a fare-thee-well than by getting a hex edit wrong. Always, always, and without any exception whatsoever, keep a backup file. And do this every time you change it.
