Is there a way to prevent Outlook from changing the meeting end time when I change the start time?

Solution 1:

Yes, you can change the start or the end times which will in turn increase or decrease the meeting duration, but you have to do it in the calendar view.

Just click on the meeting, and start dragging from the corners:

If you want more granularity while choosing the times (like 5 or 15 minutes), you can increase the time intervals shown on the calendar view by right clicking in the time line, and choosing an appropriate detail level BEFORE modifying your meeting:

More reading here.

Solution 2:

If you change the start time in Appointment window, it will assume a set duration, and will adjust the end time accordingly.

However, if you click on the Scheduling Assistant, you will be able to drag the start time line backwards and forwards in 30 min intervals, which will leave the end time unaffected. Caveats: if you try to move the start time to within 30 mins of the end time, it will push the end time ahead by 30 mins; in addition, I can't see a means of adjusting this 30 min window (say to 15 mins).

In your scenario, this would not help you, but if you wanted to reduce a 2hr meeting to 90 mins by starting 30 minutes later, this technique would work.