You could try editing it in the database. You'd want to edit the record in table datastore where title="options". What's stored in that field is a serialized array, so if you find and edit it you also have to fix the length. So it might say (somewhere in the middle of a long string)
Code:
s:5:"bburl";s:21:"http://mydomain/forum";
and if you change it to someotherdomain.com, you'd also have to fix the length, so you'd end up with:
Code:
s:5:"bburl";s:28:"http://someotherdomain/forum";
If you do this and get it working, the first thing you'd want to do is go to the options and change it there (it will still look like the old value because the datastore was changed but not the settings table).
There's a tools.php that you can upload to your admincp folder and it lets you fix some things, but I can't remember if the url is one of the things it lets you fix.