Here's a regex that might work for you.

$cn = 'CN=Admin-Lastame\, Paul,OU=Users,OU=Data_Admins,OU=Admins,DC=....'

$pat = 'CN=\w+\-(\w+)\\,\s+(\w+),.*$'

[regex]::Replace($cn, $pat, '$2 $1')

It returns Paul Lastame

Edit: just reread your op. rewritten for just firstname lastname

Edit2: I knew that original regex wouldn't handle two word last names like St. James or hyphenated names which was why I said "might work". Anyway while playing golf with the regex I modified it to handle hyphens and two word last names. If you don't have those kind of names the original works. Replace with the pattern below if you do.

$pat = 'CN=Admin-(\w.+)\\, (\w+),.*$'