Just booked my excursions - first time on Regent. I couldn't find this tip anywhere - sorry if double posted.
You will need the Name of the Excursion to find it among the list of available ones. I only had the number and it was a challenge. The number does appear in the details when you select it (helpful as some of the titles had changed).
Once you select it and confirm it you can no longer change it online - a call to Regent or TA is needed.