John Ottman has said that he regrets that he often doesn't get a chance to do a follow-up/sequel to a previous film's score to "push motifs further" as he put it. He had a chance to do it with the Fantastic Four
movies. That said, I don't believe he is going to completely abandon the First Class
theme because it's popular. I can see him weaving that in for scenes set in 1973 with the younger mutants.
But I believe his MAIN theme will be the one he introduced in 2003 for X2: X-Men United
And he should. Opening the film with it will be amazing and nostalgic, putting audiences and fanboys in a certain mood of anticipation of what's to come, considering what's already come before.