Creating a Master Schedule

I will make the assumption that you have inserted the sub-projects as read/write.

If you have tasks within the Master schedule as well as inserted projects, and you make changes to both the tasks within the Master Project as well as tasks within the inserted project, and then you click on save, it will first ask you if you want to save your Master project. If you select Yes, it will save the changes that you made to the task(s) within the master project. MS Project will then ask if you want to save changes to the (specific) inserted project. If you select No, no changes will be made to the inserted project.

Hope this helps!
J Black
www.pmconnection.com