As the creator/owner of an app, you can make changes to the app definition at any time. These changes take effect immediately. For example, if you add a change workflow rule to your app, it starts to work the moment you save the change.
Most of your changes to the app definition involve changes in the actual mobile app. These changes are automatically picked up by your users the next time they Sync their app from their mobile device.
As described in the section on Sync, it is possible that an update to your app definition can disrupt your app users. So you should be thoughtful in making certain app updates. Here's the scenario. Your app user has one version of your app and captures some new data. Before they Sync the data, you make a change to the app definition (let's say you change some columns in the spreadsheet and regenerate the app). Now when your user tries to Sync the change, there is a mismatch and AppSheet shows the user an error message.
At this point, the app is "stuck" -- it cannot fetch the latest definition of the app unless it is able to send its changes to the backend, but the backend is unwilling to accept the changes because they cannot be applied! The only remedy is for the user to discard the changes (via the Reset Changes menu option). If the number of changes is small, the user can first click on the Show Changes menu option and email the changes to you or someone else to manually apply them. If the number of changes is large and it is really crucial that this data be retrieved, please contact us and we'll consider a custom approach to retrieve the data.