SERVERA
SERVERA\INST
SERVERB
...

cls
# load assemblies
[Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.Smo")
[Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.SqlEnum")
[Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.SmoEnum")
[Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.ConnectionInfo")

$userName = "sqldba"
$password = "P@$$w0rd"

# file errors will get sent to
$errorfile = "C:\errors_ChangeDBOwner.txt"
New-Item -itemType file $errorfile -force | Out-Null

# file output will get sent to
$outputfile = "C:\ChangeDBOwner.txt"
New-Item -itemType file $outputfile -force | Out-Null

# loop through all SQL Servers
$servers = Get-Content "c:\AllServers.txt" 
try
{
	foreach($srvname in $servers){
		$srv = New-Object Microsoft.SqlServer.Management.Smo.Server ($srvname)
		$srv.ConnectionContext.NonPooledConnection = "True" 
		# login using SQL authentication, supplying the username and password
		$srv.ConnectionContext.LoginSecure=$false;
		$srv.ConnectionContext.set_Login($userName)
		$srv.ConnectionContext.Password = $password

		$sa = $srv.Logins["sa"]
		foreach($db in $srv.Databases)
		{
			if(!$db.IsSystemObject)
			{
				$db.SetOwner($sa.Name)
			}
		}
		$srv.ConnectionContext.Disconnect()
	}
}
catch{
	$err = $Error[0].Exception
		while ( $err.InnerException )
	    {
		    $err = $err.InnerException
		}	
	    $srv.Name + " - " + $db.Name + " :  " + "TRAPPED: " + $err.Message | Out-File -append -filePath $errorfile
	$err
}
	

/*  http://thelonelydba.wordpress.com
-- list bases owned by specific login. script to change below.
SELECT name, SUSER_SNAME(owner_sid) owner
FROM   sys.databases
where SUSER_SNAME(owner_sid) = 'user_to_remove'
*/
DECLARE @sql nvarchar(4000);
DECLARE @BigSQL nvarchar(4000);
DECLARE @dbName varchar(100);

declare cbases cursor fast_forward for
 SELECT name
 FROM   sys.databases
 where SUSER_SNAME(owner_sid) = 'user_to_remove'
open cbases
fetch next from cbases into @dbName
while @@FETCH_STATUS = 0
begin
 SET @sql = N'exec sp_changedbowner ''''sa''''';
 SET @BigSQL = N'USE [' + @dbName + ']; EXEC sp_executesql N''' + @sql + '''';
 --print @BigSQL
 EXEC (@BigSQL)
fetch next from cbases into @dbName
end
close cbases
deallocate cbases