How to change output format in Clocked Video Output

I haven't actually done what you are trying to do, but based on the documentation and other VIP experience, I have a couple ideas:

1. Are you clearing the GO bit, waiting for the STATUS bit to be cleared and then setting the GO bit again? (See the Avalon-MM Slave Interfaces section of the VIP document for some more details).

2. The CVO supports multiple banks of output configurations. If you have a 60Hz configuration before the 50 Hz one, you must disable the 60Hz configuration for it to use the later one.