It may help to clarify whether you're upgrading to PayPal Payments Pro vs. PayPal Payflow Pro.
If you'll be using PayPal Payments Pro, then there are no credentials to enter at our end, aside from your PayPal Email; the API permissions in our setup instructions would take care of the rest:
If you'll be using Payflow Pro, that uses the same PayPal Manager credentials as PayPal Payments Advanced:
In both cases, card-based checkouts would use a secure checkout page we manage for you, rather than anything at PayPal's end. To preview what this page looks like, you can test the demo cart buttons at either of the links above. It's mostly the same as the buyer name/address initial checkout page you saw with PayPal Advanced, except that it incorporates card account info fields in the same page, rather than using the separate, bare-bones PayPal-hosted card-info page you saw with Advanced. BTW, we're quite aware that checkout page looks rather dated and are planning to modernize it soon. :^)