Mad, they will remain the same, but will just be migrated to a CD Projekt Red account. It sounds to me like this will be only for CD Projekt Red Cross Progression, My Rewards and the RED forums. To be honest, since I don't use those things, I find it all a bit confusing.
Your GOG account and all your GOG purchases remain unaffected. The GOG.com website and the GOG Galaxy app will continue to use the GOG account system. Your GOG account will be separate from your CD PROJEKT RED account.
If you choose to opt out, please note that on March 5th, 2024 you will lose access to Cross Progression, My Rewards, and RED Forums, and all connected data will be permanently deleted.