Thank you for posting in Microsoft Q&A forum.
Adding a new site system server like MP, DP to an existing boundary group that is managing current primary site clients should not impact the clients at the ABC site. As per Microsoft, you can add site system servers from any site from the hierarchy.
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Add comment".