CPU saving way to lower quality of Theora stream

Hi,

is there a good concept to lower the quality of a Theora stream without 
completely reencoding it?
For example to make a lower bitrate stream out of a higher bitrate 
stream on the fly on some sort of proxy server. I am thinking about 
something like requantization.

If you think requantization is also the best (or the only) option for 
Theora please let me know.

Kind regards,
Franz