Segue script que sempre utilizo para mudar o dbowner dos meus bancos quando necessário.
SELECT 'use '+QUOTENAME(name)+ ' exec sp_changedbowner @loginame =''sa'' ' ,suser_sname(owner_sid)
FROM sys.databases AS d
WHERE [name] NOT IN ('master','TempDB', 'msdb', 'model','DWConfiguration','DWDiagnostics','DWQueue')
AND [name] NOT LIKE 'ReportServer$%'
AND suser_sname(owner_sid) <> 'sa'