Great argument. The paradox changes that variable, possibility of no execution, midway through.

#1 If it stays true from the beginning, the first step of the prisoner's logical induction is invalid and the prisoner accepts he will be surprised.

#2 If it stays false from the beginning, the prisoner can make the logical induction, coming to a conclusion that is invalid (contrary to an assumption) so he accepts that he will be surprised.

If it can change at any point, see #1.