I have a list of folks and their DN from AD (I do not have direct access to that AD). Their DNs are in format:
$DNList = \'CN=Bob Dylan,OU=Users,OU=Dept,OU=
I decided to turn my comment into an answer:
$DNList | ForEach-Object {
$_ -replace '^.+?(?

Debuggex Demo
This will correctly handle escaped commas that are part of the first component.
We do a non-greedy match for one or more characters at the beginning of the string, then look for a comma that is not preceded by a backslash (so that the dot will match the backslash and comma combination and keep going).