I found this great Powershell script that convert virtual disk files to eagered zeroed.

Awesome script! I had to make some changes to make it work in my environment.

 function Set-EagerZeroThick{
 param($vcName, $vmName, $hdName)

# Find ESX host for VM
 $vmImpl = Get-VM $vmName
 Write-Host $vmImpl
 if($vmImpl.PowerState -ne "PoweredOff"){
 Write-Host "Guest must be powered off to use this script !" -ForegroundColor red
 return $false
 }

 $vm = $vmImpl | Get-View
 Write-Host $vm
 $esxName = (Get-View $vm.Runtime.Host).Name
 Write-Host $esxName
# Find datastore path
 $dev = $vm.Config.Hardware.Device[10] | where {$_.DeviceInfo.Label -eq $hdName}
 Write-Host $dev
 if($dev.Backing.thinProvisioned){
 return $false
 }
 $hdPath = $dev.Backing.FileName
 Write-Host $hdPath

# For Virtual Disk Manager we need to connect to the ESX server
 $esxHost = Connect-VIServer -Server $esxName -User $esxAccount -Password $esxPasswd

# Convert HD
 $vDiskMgr = Get-View -Id (Get-View ServiceInstance -Server $esxHost).Content.VirtualDiskManager
 Write-Host $vDiskMgr
 $dc = Get-Datacenter -Server $esxHost | Get-View
 Write-Host $dc
 $taskMoRef = $vDiskMgr.EagerZeroVirtualDisk_Task($hdPath, $dc.MoRef)
 Write-Host $taskMoRef
 $task = Get-View $taskMoRef
 Write-Host $task
 while("running","queued" -contains $task.Info.State){
 $task.UpdateViewData("Info")
 }

 Disconnect-VIServer -Server $esxHost -Confirm:$false

# Connect to the vCenter
 Connect-VIServer -Server $vcName
 if($task.Info.State -eq "success"){
 return $true
 }
 else{
 return $false
 }
}

$vmName = "vm1"
$vCenter = "vcenter_server"
$esxAccount = "rootuser"
$esxPasswd = "password"

Set-EagerZeroThick $vCenter $vmName "Hard disk 1"

To validate before and after running the eageredZeroed script I run the following script:

$vm = Get-VM vm1 | Get-View
$vm.config.Hardware.Device[10].backing
$vm.config.Hardware.Device[10].DeviceInfo

Source: http://www.lucd.info/2009/11/15/scripts-for-yellow-bricks-advise-thin-provisioning-alarm-eagerzeroedthick/