As far as I'm concerned I like to follow the MOM on amount, it's not supposed to fill the entire area, it's just a fluid built to help the shaft drive. As far as brake fluid goes I've never had to replace all of it. I can't remember if you're always supposed to have some in there or not?
So you have quite a few options.
1. remove some fluid and replace some until you feel it's all better
2. Remove all the fluid, once again, I've never had to do this but you could probably just get all the fluid out by bleeding the system dry, except you're not removing bubble you're removing all the fluid