I presently have my site set up to allow members to pay their fee using PayPal. How difficult would it be to change that from PayPal to using Stripe instead?
You shouldn't need to -- the API Keys are only used with their respective APIs. Meaning that the Paypal keys will not be used with Stripe, and Stripe keys won't be used with Paypal. All of the settings are stored in the database associated with OSSN; meaning that it's just an entry. Personally, I would leave them, because if you choose to switch it over / back to paypal for some reason (like, something happens to your stripe account, it goes down for some reason, etc.) it'll be there and readily available. And, it staying there doesn't hurt anything.
I have now registered with Stripe and added their API keys. I notice in my Configure, that even though I have selected Stripe, my PayPal API keys are still in place as well. Should I delete the PayPal, or is it not going to interfere with the Stripe operation? Also trying to figure out how to get the major credit card images on that Paid Access page.