I was hired to do some web design modifications for someone's site and after that they asked if I could also help them switch their CC payment processor for their site members. Seemed easy enough given they had the API code to use from the new processor. But now I'm realizing it's not easy.
1. They have recurring payments for some members and I'm not sure how those will continue if they switch to a new one. Even if the previous processor continues to rebill them, how will they cancel/update their info if the site now processes through a different company?
2. They are using ModX and AMember for their site and it is very modular so it's hard to piece everything together. And their current processor, Stripe, is added through a plugin in the manager. Their new processor doesn't have a plugin so I'm trying to modify the Stripe one I copied from their server backend to implement the new API code but I'm not having much luck. The plugin is written in PHP. I'm fairly decent with PHP but this all appears to be over my head.

Any suggestions/tips from similar experiences? I'm afraid I bit off more than I can chew taking the extra work.