In the current ACS Network, your opt-in/opt-out status is managed on an acs.org system which many of you access through My Account https://myaccount.acs.org/myaccount/AcctOverview.html . You are able to toggle between opting in and opting out without even going to the ACS Network.
After the migration, this will no longer be the case. Users will need to actually come to the ACS Network and set up their account. Accounts will no longer be created passively via acs.org (e.g. MyAccount). When you first come to the ACS Network after the migration, you will login with your ACS ID, then you will be asked to accept the Terms and Conditions and to set up a user display name, i.e. the name that shows up on your profile and with all of the content you post.
With this change, opting out of the Network will be somewhat different. Instead of just opting out, you will delete your account on the Any content that exists in your name when you do this will be anonymized. If you change your mind within one week, you may contact support and they will help get your account reinstated, i.e. all of your content re-associated with you account.
After a week, the deletion becomes permanent. You may rejoin the ACS Network, but you will need to create a new account. None of your old content can be re-associated with this account. You may use the same user display name if it is available.