Move content between filestream containers
According to Paul Randal (who is as reliable as they come) there is no simple way to do this and you essentially have to recreate the table in a new filegroup that is pointed to your new filestream location.  The link is there so you can see for yourself, but this looks like the only real alternative.  It is not an ideal one, obviously but it should get you the end result that you need.   But not in a simple fashion, I am sorry to say.