/*
  1. set master DB as the context database
  2. Give the target template path (use only template specific subpath)
  3. Parent folder path
  4. ($_.TemplateName -eq "Image") - name of the existing template in quotes.
*/
$master = [Sitecore.Configuration.Factory]::GetDatabase("master");
$entryTemplate = $master.Templates['System/Media/Unversioned/Image'];
cd master:"\media library\Documents";
Get-ChildItem -recurse | ForEach-Object { if ($_.TemplateName -eq "Image") { $_.ChangeTemplate($entryTemplate) } };

///The following code changes the template of 'Versioned' Pdf files to 'Unversioned' Pdf files.
//$master = [Sitecore.Configuration.Factory]::GetDatabase("master");
//$entryTemplate = $master.Templates['System/Media/Unversioned/Pdf'];
//cd master:"\media library\Documents";
//Get-ChildItem -recurse | ForEach-Object { if ($_.TemplateName -eq "Pdf") { $_.ChangeTemplate($entryTemplate) } };