@ravinder singh,Thanks for posting in Q&A.
From your description, I know you want to know what the impact on existing enrolled device without managed apple id will be once you enable federation in ABM and enrollment without user affinity.
Based on my researching, there will not be important impacts for existing devices, they will continue to work as usual. However, if you want to disable Sharing mode, wiped and re-enrolled, it will require a Managed Apple ID. If you do not have a Managed Apple ID, you will not be able to enroll the device.
Hope this can be helpful.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.