Thank you for your post.
Generally, you shouldn't make schema changes to a file that is hosted and accessed by other users. Send a message to each user to log off while the changes are made. You are modifying the structure of a database, much like you would be modifying the structure of a building. With a building, you would be asking tenants to move out until the construction is complete.
This worked fine in FileMaker 9, but it doesn't work now in Version 10.
Here's a movie with the problem:
I posted the same problem earlier today, but I wasn't aware the problem was submitted here...
This problem was reported about a year ago, but we were unable to duplicate the problem in house. I have attached this thread, as well as the other thread posted by "CampSoftware", to the original case, and our Development and Testing departments will be researching this further. I will post again when more information becomes available.