Change Site Collection Administrator

$webappurl = "https://portal"

$account1= "domainname\username"

$account2= "domainname\username"

$webapp = Get-SPWebApplication $webappurl

$allsites = $webapp | get-spsite -limit all

$allsites | foreach-object { Set-SPSite -owneralias $account1 -identity $_.url}

$allsites | foreach-object { Set-SPSite -secondaryowneralias $account2 -identity $_.url}