I've switched a few times from S to D and back to S all while driving with no issues. Should be fine to do that while moving.
In S as soon as you start to use the paddle shifters, you then need to continue using them until you switch it back to D, also I'm pretty sure you can hit the S button again (while already in S) to "reset" it and it will be back in sport automatic mode.
You can use the paddle shifters in D to temporarily go up or down a gear and the transmission will adjust after a short time if your speed is too high or low.
In S as soon as you start to use the paddle shifters, you then need to continue using them until you switch it back to D, also I'm pretty sure you can hit the S button again (while already in S) to "reset" it and it will be back in sport automatic mode.
You can use the paddle shifters in D to temporarily go up or down a gear and the transmission will adjust after a short time if your speed is too high or low.