pre purchases shore excursion

If you pre purchases shore excursion can you cancel and exchange them once onboard?

The reason I'm asking is we already purchased excursion for our up coming cruise and now some of our group is thinking that they want to do a different one.

The new one is also cheaper so if they allow exchange or cancel is the balance given as OBC

I believe you may cancel up to 48 hours before the excursion.