You cannot update graphics documents or pages in read-only projects. (You can, however, update pages in a read-write project if that top-level project includes one or more read-only projects.)
Opening an updated graphics page in a read-only project in Graphics Builder will attempt to show the upgraded symbol, but only at the presentation (not disk) level.
Note: Updating graphics documents and/or pages in mid-level include projects is not recommended because it will not update pages in the top-level project. However, issuing an update pages at the top level iterates through included projects recursively. (If these need to be updated, make the relevant projects read-write first).