I would assume that they will be followed till the conclusion of the study. Even the subject who declined having a second colonoscopy might allow less invasive followup.

An implication of this proof of concept study is that they have to increase sample size to make up for subjects who are not willing to consent to that second colonoscopy. I wonder if they get more than two, remembering just how "pleasant" a colonoscopy is (EDIT: even on an every 10 year basis).