Study expanded from three to five rounds: How to write this down?
Just describe what actually happened like you did to us. Also explain why this happened. There is no reason to slavishly adhere to some style that doesn’t fit special situations like yours.

In general, your thesis (and any other scientific publication) should contain everything needed to reproduce and assess the results. As the circumstances you describe are certainly relevant to this (e.g., participants may drop out of the study after prolongation), you cannot possibly leave this unmentioned.