This is a script you can use to re-register multiple templates to your vCenter's inventory. It will simply get a list of templates, their folder location, host, etc, remove it from inventory and re-add it back exactly where it was.

Reregister_VMTemplates.ps1

<#
Re-register templates that are locked
KB Reference: https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2037005
#>
Import-Module vmware.vimautomation.core
Connect-VIServer NameofyourVcenter

$TargetTemplates = Get-Template "NamePatternOfTemplateOrRemoveThisIfYouWantToTargetAllTemplates*" -server $Global:DefaultVIServer
Foreach ($Template in $TargetTemplates)
    {
    $DatastorePath = $Template.extensiondata.summary.config.VmPathName
    $Location = Get-Folder -id $Template.folderid
    $VMHost = Get-vmhost -id $template.hostid
    $TemplateName = $Template.Name
    Remove-Template -Template $Template -Confirm:$False
    New-Template -VMHost $VMHost -Name $TemplateName -Location $Location -TemplateFilePath $DatastorePath -Confirm:$false
    }