Originally Posted by DiscoP:
“I'm actually quite surprised that there seems to be so much demand for McGann to appear in the episode.”
I think there is some interest even from fans who joined with Nu Who. However, for most viewers it is not the actor McGann that needs to be respected, rather the continuity itself. McGann could be replaced or his Doctor's changed appearance explained away.
More importantly, John Hurt's presence creates quite a buzz about the show, appealing to possible new fans.
I'm going to give Moffat credit in advance that he will tie the Eighth Doctor nicely into continuity somehow, perhaps a brief flash of McGann, aging him or regenerating him into Hurt.