D&D Beyond has said that Monsters of the Multiverse will not replace existing monsters already purchased by users.
While they have indicated that existing content will not be overwritten, they were unable to share any details on how the new monster stat blocks will be implemented - suggestions might include duplicate entries, or some kind of toggle. This also includes racial traits, which won't replace old material -- the contents of the book will be treated as new content.
While DDB is taking it's lead from WotC on what to do, apparently WotC asked them to take charge of communicating this all to users.
While they have indicated that existing content will not be overwritten, they were unable to share any details on how the new monster stat blocks will be implemented - suggestions might include duplicate entries, or some kind of toggle. This also includes racial traits, which won't replace old material -- the contents of the book will be treated as new content.
While DDB is taking it's lead from WotC on what to do, apparently WotC asked them to take charge of communicating this all to users.