The feature your interested in is called portable identities and is still in development, as far as i know. As far as matrix is concerned there is no difference between a personal chat and a group chat. There are rooms with members, thats it. Maybe the tool can kick your old identity after a successful transfer though. Hope this helps a little :)

You can migrate your account using this tool from Element: https://ems.element.io/tools/matrix-migration

Yes, it does make your DMs be under the rooms section, but you and your friends can input the command /converttodm into Element's web/desktop client (on mobile, it doesn't work as far as I know), and you're back to normal.