^It would cost thousands of dollars to replace, it doesnt affect the track, just the supports around it (hence the rock) and the rock has lasted around 20yrs so it wouldnt have to be "constant", but it wont happen.
Are you sure the rock lasted 20 years because if you look at the picture here and the picture from the earlier demon days and the present rock, it's a lot different. Maybe I'm mistaken.
The picture in your first link appears to be the Demon located at Santa Clara and the second picture the Demon at SFGAm.
http://www.rcdb.com/ig76.htm?picture=1: this picture shows a front view of the demon's mouth from Santa Clara. It's copyright is 2000. The second link you provided has a date of 1999.
So, one can assume that the two demon's on the Demon are different at each park, or whoever put the copyright on the pictures did not do so correctly.