CarolineS The migration will happen sometime after both your stage and production environments have been upgraded to 18.8. Unfortunately, it's hard to comment on a specific date for that. Once the migration has been done, you will see an entry in your Studio>Versions that reads "freemarker asset.get wrapper migration". If your skins do not use it then there will be no entry. For testing, just make sure that if you use any assets like background image, avatars, or any other icons in your skin then they are rendered properly on your community.
Also, these changes are only for skin files. Components and other freemarker files continue to work the same way.