If you check the box to write changes to XMP, it will eventually do it for all files you have edited, not just ones edited since you checked the box.
If you want it to do the writing right now, make sure you’re in the Library, then click on the “Metadata” menu at the top of the screen. Almost all the way down at the bottom, there is an option that says, “Save Metadata to File”. I presume that will do so for all photos that are selected at the time that you run that option.