Atualize a tabela do banco MSDB:
/*Here’s how to change the owner of a maintenance plan to dbo in SQL Server 2005*/
— to find the name and owner of the maintenance plan
— select * from msdb.dbo.sysdtspackages90
— to find the sid you want to use for the new owner
— select * from sysusers
UPDATE
[msdb].[dbo].[sysdtspackages90]
SET
[ownersid] = 0x01 — SA
WHERE
[name] = ‘MaintenancePlan’
http://dbaspot.com/forums/sqlserver-server/356339-change-maintenance-plan-owner.html