Each time you make a payment you should be able to decide your method of payment for that particular transaction. After you put in the amount and the e-mail where you're sending it, there should be a section on the screen marked Payment Method where you can choose to use a credit card or linked bank account, or you can use your cash balance in Paypal if you've received some payments. I not sure how you permanently change the default to always be credit card.
I haven't ever given Paypal my bank account, just a credit card. I figure if my account information is compromised, my credit card offers some fraud protections, and I can always cancel my card.
Once they have you bank account - you always have to change the transaction and move it over to credit card... I only buy with the card but i have my account linked for when I sell and if I ever made anything it could drop in over there. Never happened but that was the dream at least.
Yes - above the spot where they tell you how you are funding it says something to the effect of "change funding source" click there and pick what ever you like. I have had to do this every time and yes I do it for the protection the cc gives me for online buying. I learned early on that paypal does nothing when it comes to a issue of refunding in reality my cc is my back up plan... paypal first, when that fails then I have the added layer of my card.