Yes it will break and I may have to rebuild the farm.
SharePoint is part of many local groups and they will no longer be recognized when you demote the DC. ex: WSS_WPG , WSS_ADMIN_WPG, WSS_RESTRICTED_WPG
I am pretty sure dbcreator and securityadmin server roles will not be recognized as it was registered with AD (Farm)account ..
After fixing everything .. you will still end up doing the PS config and you may not pass the step 6
😊 …