For some reason the Grid line is not snappable and it's properties can't be changed, in addition the message center displays "Change Rejected in order to preserve design integrity".
There is an option to make it snapable, do an "Element information" on the Grid and in the "Extended" tab Select the Snappable drop down menu and make sure that it is set to "Snappable". If still unable to snap then this is because the master file contains associative dimensions that are dependent upon the referenced elements/files and it is the reason for the message "Change Rejected in order to preserve design integrity".
If the file or element that you are trying to manipulate is referenced then you might be able to apply the changes by either exchanging the file or activating it.