Its not impossible, but its VERY DIFFICULT to do it.
It would be time consuming + expensive + have an adverse effect on the housing recovery. Thus such a replacement is easier said than done.
Its actually easier to leave FnF with some modifications (backstop govt/private mix, stricter loan norms, etc) IMO.
Obama used the word "wind down" which could mean a lot of things (see my post history, I have all the terms that it could mean in a post), which includes 'restructure'. Media is pushing the word 'shut down' for obvious reasons. Thus Obama left a lot of room for himself to work with.