Yes they are very clever and hide how many spots are left during these excursion sales. We just had one last week for Alaska cruises. I guess this week is Northern Europe. You should be able to cancel and then immediately rebook it, it should say "Apply Credit" or something like that instead of a pay button, then you should show as rebooked in the excursion at the lower price, and the refund of the price difference posts to your credit card within 2 to 3 days. I don't use OBC to pay for excursions because I don't know how smooth the refund process is, so if you used that instead of paying on a cc then I don't know how it works.