Are they running Paya with the default install that comes with Sage or with the advanced install of Paya which has additional functionality? If it is out of the box, there isn't anything to uninstall. The other integration either uses parts of it or overrides it as needed. If it is the additional functionality, it would be like any 3rd party modification. As in it can get tricky and your mileage may vary from others. Your Tokens will not come over unless they do a Gateway to Gateway transfer which I have seen happen, not with Century but with others. We use to have a program that used Paya's API to get the token and the full card but they closed that when they got rid of the vault only accounts.
Reverting back is the same type of process, including with the Tokens.
If they want to test, it will need to be in a test install not a test company. The modifications are system wide.
Is e-Commerce the old B2B or B2C with Sage or something external like WooCommerce or Magento, etc? If the old eBusiness B2B or B2C then you might have issues with Century working correctly. If it is something external than just make sure they have already worked with century to get the orders in and see if the CC is passed in or not. Depending on the integration to eCommerce they may or may not be passing all the info to do anything with the CC once it is in Sage.
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Todd Martin
MBA Business Software
https://www.mbabsi.com------------------------------