Change template without breaking layout

ChangeTemplateMaintainingLayout.ps1

$item = Get-Item -Path master: -ID "{C7FF7164-CECC-44A5-9A16-784619578316}"

$sharedLayout = $item.__Renderings
$finalLayout = $item."__Final Renderings"	

if ([Sitecore.Xml.Patch.XmlPatchUtils]::IsXmlPatch($finalLayout))
{
    $finalLayoutFull = [Sitecore.Data.Fields.XmlDeltas]::ApplyDelta($sharedLayout, $finalLayout)
}

## Change template
Set-ItemTemplate -Item $item -Template "/sitecore/templates/User Defined/NewTemplate"

# Reget item
$item = Get-Item -Path master: -ID $item.ID

## Layout will be broken now, so work out a new patch delta between the finalLayout and the new sharedLayout
if ($finalLayoutFull) {
    
    $delta = [Sitecore.Data.Fields.XmlDeltas]::GetDelta($finalLayoutFull, $item.__Renderings)
    $item."__Final Renderings" = $delta
}