Attribule all content of users being deleted to the new one when 200000 users

Hello everybody.
Please help with the following issue.
I need to add new Administrator and delete the old one in WP. But the problem begins when I try to attribute all content to new user: there are 200000 registered users and tab in browser just freezes while trying to build a select with so many users I suppose. Maybe someone knows how to overcome this.

TNX.