Can Mark change a campaign's objective after it's created?

No. A campaign's objective cannot be changed after it is created. This is a Meta rule, not a Mark limitation.

Why Meta locks it

The objective determines how the entire campaign is optimized and how its results are measured. Meta builds the campaign's delivery around that goal from the start, so it locks the objective at creation to keep the data consistent.

What to do instead

If you need a different objective, create a new campaign with the right one. Mark can set it up quickly, and it can reuse the same audience and creative, so you are not starting from scratch.

What you can still change

Everything else on an existing campaign is editable at any time: the budget, the schedule, the audience, and the creative. Only the objective is fixed, so it is the one choice worth getting right up front. If you are unsure, describe your goal and let Mark recommend the objective before it builds.