Update-Files.ps1

<#
.SYNOPSIS
Copies specified source files that exist in the destination directory.

.INPUTS
System.String of paths to copy from, if matches exist in the destination.

.FUNCTIONALITY
Files

.EXAMPLE
Update-Files.ps1 C:\Source\*.txt D:\Dest

Copies *.txt files from C:\Source to D:\Dest that exist in both.
#>

#Requires -Version 3
[CmdletBinding(SupportsShouldProcess=$true)][OutputType([void])] Param(
# Path(s) to copy files from, wildcards allowed.
[Parameter(Position=0,ValueFromPipelineByPropertyName=$true,ValueFromPipeline=$true,Mandatory=$true)][string[]] $Path,
# Folder to copy files to, if they already exist there.
[ValidateScript({Test-Path $_ -PathType Container})][Parameter(Position=1)][string] $Destination,
# Indicates files should only be copied if they are newer.
[switch] $NewerOnly
)

Process
{
    foreach($file in Resolve-Path $Path)
    {
        $destfile = Join-Path $Destination (Split-Path $file -Leaf)
        if(!(Test-Path $destfile -PathType Leaf)) {continue}
        if((!$NewerOnly -or (Test-NewerFile.ps1 "$destfile" "$file")) -and
            $PSCmdlet.ShouldProcess("'$file' over '$destfile'",'copy'))
        {Copy-Item $file $destfile}
    }
}