Powershell to replace/rename multiple files in a directory

Plus one for WhatIf, sensible plan :)

Weeeeeeell ....
Chris, sorry, but just to avoid confusion in case somebody tries to run your script, I don't entirely agree with "Both examples will work to rename items., as on closer inspection, I see two errors in your script.
I'll spoil it for you:
* You're not using a ForEach, so there's no "$_" variable
* You wrapped the new name in curly brackets (turning it into a script block) instead of round ones

It's not often you're less then spotlessly perfect oBdA, but on this occasion I'm afraid I've caught you out.

When processing a pipeline commands which fill parameters from a pipeline can be fed alternate values using the technique above. It *only* works because of this:

Get-Help Rename-Item

-Path <string>

    Accept pipeline input?       true (ByValue, ByPropertyName)

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In this situation the value supplied for a parameter in the next command in the pipeline must be described by a script block. The script block will be invoked for each iteration of the output pipeline.

Test code to play with:

New-Item C:\Temp\TestSet -ItemType Directory -Force
1..10 | ForEach-Object { New-Item "C:\Temp\TestSet\$_.txt" }
Get-ChildItem C:\Temp\TestSet -Filter *.txt | Rename-Item -NewName { "$($_.Name).new.txt" }	

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There is no problem with the answer I've given above.

OK, my bad, sorry, I retract that statement, your script works just fine,
When I first tested it, it showed the expected error, so something must have been wrong, because now I can't reproduce the error anymore, of course.