Hi Olivia,
To understand the "At Risk" change, I'd suggest you navigate to the Tasks view on the Project, turn on the Gantt chart, and turn on the option to see the Projected Dates. By doing so, you can visualize how the Progress Status changes (On Time, At Risk, Late) as Projected Dates match and extend past their Planned Dates...particularly when you type in a "Must Finish On". Continuing to observe, recalculate the Project timeline, then notice how doing so might affect both the timing (e.g. in some cases "snapping" Tasks into new positions) and by extension, the Progress Statuses.
Regards,
Doug