Use Powershell to replace subsection of regex result

You need to group the sub-expressions you want to preserve (i.e. put them between parentheses) and then reference the groups via the variables $1 and $2 in the replacement string. Try something like this:

$regexA = '( TEST=[A-Za-z]+)14(\d\d)$'

Get-ChildItem '*.txt' | ForEach-Object {
    $c = (Get-Content $_.FullName) -replace $regexA, '${1}16$2' -join "`r`n"
    [IO.File]::WriteAllText($_.FullName, $c)
}

Here's an example using a scriptblock delegate (sometimes called an evaluator):

$regex = [regex]'( TEST=\D+)14(\d{2})\s*$'
$evaluator = { '{0}16{1}' -f $args[0].Groups[1..2] }
filter set-number { $regex.Replace($_, $evaluator) }

foreach ($file in Get-ChildItem  "*.txt")
 {
   ($file | get-content) | set-number | Set-Content $file.FullName
 }

It's arguably more complex than the -replace operator, but lets you use powershell operators to construct the replacement text, so you can do anything you can put in a script block.