This is exactly what they're doing. According to the announcement, photos uploaded in "high quality" (the previous free/unmetered tier) before June 1, 2021 will remain exempt from the storage quota. Only new photos uploaded after June 1, 2021 count against the quota. So not only are all the existing photos and videos grandfathered in, you even have seven whole months after the change was announced to upload new material which will also be grandfathered in.
Google is also very good about data migration; you can download all your photos and videos along with a wealth of other account data in reasonably open formats via Google Takeout and migrate it wherever you wish.