@TomWijsman has given a great answer regarding make a true Metro tile
From another perspective, if you just want to update the icon to something Metro-ish for an app, you can just change the icon:
- Right click on the tile and select "Open File Location"
- Right click on the shortcut and select Properties
- Click on the "Change Icon" button
- Select a new icon to use
You can google for metro style icons if you want to replace one in an existing app.
I'm sure there'll be some web app that shows up some enough that'll let you take an existing icon and make it graphically Metro-ish, but I couldn't find one yet.