Most would recommend to google FileMaker Separation Model, or search this site for separation model. Then you can make your changes and not affect underlying data (unless of course you actually remove/add fields, calculations, etc.).
You could also make you changes on the development side, and import your data into the new version, checking both versions for data integrity.
Just remember to constantly back up you builds. The amazing PhilModJunk has a script to automate back ups of a build. If he doesn't reply with a link, I'll dig it up later today.
Perfect, thanks Guys.
I've been working very carefully on new versions of layouts today, making sure not to delete or change any existing fields or tables. Seems to be going OK without causing problems for anyone.
Definitely going to enable the auto-back-up.
Editing The Live Database:
Do it all the time, but it's risky. Layout, value list and script changes are fairly safe. There is a small chance that a network glitch at just the wrong time could damage your database, bit it's a pretty small chance. The bigger risk is that your design change might catch a user "between chairs" when your changes suddenly appear on their screen in the middle of what they are doing. Incremental changes and bug fixes are usually no problem, but significant changes need to be demonstrated to the users and they need to know what changes are coming their way and when. I think of this as the "human engineering" part of the project. In my new job, I've started creating desktop videos of changes I have made to a developer copy of the database to send to representative users so that they can see what changes are headed their way and can ask questions/make suggestions before the change goes "live" in the production copy of the database.
Other changes are absolutely out of the question for a live database except, perhaps, late at night when no one is supposed to be using the DB, but even then, be careful of scheduled "housekeeping" scripts run from the server that might need to be disabled during the time you make the change:
These are changes you'd make in the Manage | Database | Window. In Server 10--haven't tried it since that version, just opening up the specify calculation dialog for a field's auto-enter calculation Locks the table an all users/scripts can't edit data in the table until you close the dialog. Any changes to the data model, such as adding new fields can lock the table down for several minutes while the changes are applied. If this happens right in the middle of a script that is modifying data in that table...
Yes, if I can only type straight!
Otherwise known as changing the spark plugs on a car while it is being driven down the freeway...