Curious little situation I came across in Revit 2013. Not sure if this happened on previous versions.
I created a roof using a mass form. Of course changes were needed so I edited the in-place mass and hit finish. I highlighted the roof but the Model by Face options were greyed out as shown in the image below.
There seems to be simple fix to work around this issue. With the roof highlighted, shape handles are available.
Dragging a shape handle will bring up a warning dialog. Just hit cancel. (if it doesn’t then just hit undo)
Now your options are available. Go ahead and click Update to Face.
Seems as though you have to give a little bump to wake thing up. Thank you Revit for keeping it user friendly