Changing any of these can have complications. If you change the merchant gateway and/or the payment processor without maintaining the current merchant gateway and payment processor, all of your current recurring gifts will cease to work. As long as those two companies stay intact, the recurring gifts will continue to work. Most of the time when clients want to change either of those two, they maintain the existing relationships in the background for existing gifts until those gifts have ended while adding new merchant gateways and/or payment processors.

For clarity on what these terms mean, review this article.