Generally, you will have to change the field "userid" in the isp_nodes table for the website.
Thats not the problem, but you will also have to change the userid in isp_nodes for all related Items (co-domains and suers). Websites and co-domains / users are linked together by tha isp_dep table. In isp_dep you will also have to change the userid to the new user.
I dont recommend to do these changes in the database as you might mess up your database.