Rot13 with numbers

I am trying to increase only numbers in a string of text ..

AB12389CD so it becomes AB23490CD

so far I have...

$a = "AB12389CD"
$b = $a[0..$a.Length]
foreach ($c in $b){
$e = [string]$c
$d = [int[]][char[]]$c
if (($d -ge 48) -and ($d -le 57)){
$e = [int]$e +1
}
}

Many thanks

K

kittuk