If you right click on your images in Bridge and do not see the option to "Generate High Quality Thumbnails" don't despair, this is most likely because your thumbnail settings are on "automatic".
Automatic does not however recognize new xmp data right away so we're going to switch it to manual and force the changes to preview.
1) Open Bridge
2) In the upper right of Bridge there is the Thumbnail options (it looks like a rectangle next to a star)
3) Click on it, change from Automatic to "On Demand"
4) When you right click on your images in the grid you will now see the "Generate High Quality Thumbnails" option