It has happened, hence their name "tentative production week." Mostly depends on whether GM's parts suppliers are they keeping their production schedule (usually, but not always 100%), compounded by whether GM, when the parts are received, is rejecting some due to quality control. If, for example, you were scheduled for TPW next week, and those with a TPW of this week had some of their parts rejected, your production could happen this week. However, GM does not notify dealers nor customers of TPW's moving up (ahead of schedule). It is rare, however, for TPW's to move forward.