I want to sign up for autopay
Have you logged into your account and looked at 'Automatic Payments' at https://customer.xfinity.com/#/settings/billing ?
You may need to 'Add Payment Method' (checking/debit/credit card) first before you can turn it on.
Do existing customers get the $10/mo discount regardless of package after signing up for autopay?
You should, but it still takess 3 months or 45 days after signing up for it to kick in.