I finally figured this out. Open the existing Calendar item you want to copy. In the lower left corner of your screen, choose the Copy icon. The existing Calendar item changes into an edit state. Modify the date in the existing Calendar item and save it. All done! Your existing Calendar item remains unchanged and you now have a copy of the existing item on the new date that you saved! Note that I tried this on a Galaxy S21+ G5, but I think it works on all recent Galaxy models.
