If you are checked in for your flight before you make a same-day change, you will lose that boarding position once you move to a new flight. You'll need to check in after you complete your change to get your new boarding position.
Once you are cleared from the same-day standby list, you will need to check in to receive a new boarding position. With same-day standby, you will not lose your boarding position on your original flight unless you are officially moved to the new flight.