Multiple copies ofa database for diff projects

I'm having a problem with versioning. I've made a few databases that get used in different projects. For example, there's one that records specific geologic information about an area. At any given time there are multiple copies of the database being used by different people who are recording data for different counties. When it's time to make some revisions on the database, however, I've got the problem of there being multiple copies. I don't want to implement the same changes in them all, so what should I do?

I had some ideas.

Currently I've just been making the changes to my "master" version, and then importing the projets' data into a clone of it and giving it back to the project. This gets to be kind of a pain though, especially if someone makes changes on his own that sometimes mess up field import order and whatnot... so this system works but it seems like there should be something better

Since FMP abstracts the layout from the actual data, can I make changes to a layout and then just drop it in an existing database? That way the projects could update the databases themselves

Should there just be ONE database that all the projects use? It would be easy to have a field (in fact there already is one) for what county/project the record is for. I'm guessing FMPserver makes this sort of thing easier, but we don't have it (and it might be too pricey for our lab to get). There's also the worry that (since most of the entry is done by student employees) someone could mess EVERYTHING up, instead of just one project/portion.

How do people manage this sort of problem?