[CentOS] /etc/pam.d/system-auth changes in update

I just did an update, and PAM was was one of the modules.

Yum has placed /etc/pam.d/system-auth.rpmnew and it's different than
the existing file, which is actually a link to system-auth-ac.

The files are slightly different, and I'm wondering if the differences
are important. I can't find any docs on the reason for the change, so
before I spend time becoming a PAM expert, I'm wondering if anyone knows
why this change was made, or were it might be documented.

Thanks,
--Bill