How to change the display ratio of a Theora track inside Ogg?
The player panic you are experiencing might be due to the checksum in the ogg header. If you recalculate the checksum for the whole header with your new value you might get a valid file again.
http://en.wikipedia.org/wiki/Ogg#Page_structure

Though I dont know all too much about the ogg and vorbis specs. For this kind of low level work its always a good idea to read the specs.
http://xiph.org/vorbis/doc/Vorbis_I_spec.html