I don't know? "None" or "a few" I'd say. There'll be a lot of moaning and bitching about the new board until everyone gets comfortable with it and the bugs are ironed out.
But if people do leave they'll be back sooner or later.
Oh, it's a make or break deal. It will alienate all the die hards, and there will be a revolution led by the relative newcomers without the old guard in power.
........Either that or it will make no damn difference cuz it's just a few changes for God's sake.
There will be some who leave. Most likely, they will have already had a bitch, or be a little bitch themselves, and will simply use this as an excuse "to show everybody!'. Yet, remarkably, life goes on.
Very few. If Facebook can emerged unscathed from jerking their customers around with incessant, idiotic design changes, Freeones has nothing to worry about.
Leaving because of a new layout though is stupid. Change happens, we are becoming more modern etc etc.
Within a week, most people will enjoy the new board and embrace it. Many people don't like change, but it happens to everyone and is part of life. You just have to accept it