I haven't change names on my excursions so take my post as such.
If you are able to change it online before your cruise, then that would be the best thing. But I'm assuming you're asking because you can't or don't want to change it online.
So, I would make sure that you take the tickets you receive for the excursion to the Port adventures desk and ask them to change the names. The only thing I can think of that may be an issue is that if you booked for a child and you are changing it to an adult. It's not just a matter of cost, it could be that there is a limit of number of children they accommodate. Some excursions do have that limit.
However, if you are changing adult to adult, I would hope there is no issue changing the name on it. The name has to match the ID for many reasons I'm sure you can imagine.
I recall a family who didn't change the name on their excursion and I think one person couldn't go so another member decided to go. Unfortunately, it was a big delay during their check in for the excursion. They require the name match the ID so I'm not sure what happened, but I didn't notice them on our excursion. I hope they were able to change it for a later time, but I really don't know what happened. It doesn't have anything to do with whether some will allow it and other won't, it's about proper ID.
