To change a discussion from multi- to single-thesis, or vice versa, open the Discussion Menu and select ‘Discussion Settings’. Click ‘Change Discussion Type’.
Note that only users with an Owner or Admin role can change a discussion’s thesis structure. You cannot change a multi-thesis discussion to a single-thesis discussion if there is currently more than one thesis in the discussion - you will need to consolidate the theses first.
For more information on discussion types, see Multi- vs Single-Thesis Discussions.