All you need to do is go to your own bank or ring them and ask for your Visa credit card to be changed to a MasterCard. They won't be a problem as long as you do it before 19th January. Until then your Visa CC is still good.
I've just spoken to Halifax's credit card department, who confirmed that if you pay with ANY credit card and Amazon is the fulfiller of the order (in other words, Amazon take the money from your account, deduct their cut and pass the rest onto the actual seller) then section 75 in the Ts and Cs of the credit card will not apply. This is if your spend is between £100-30,000. Of course, you can always plead with your provider to come to some arrangement but section 75 won't apply if things go AWOL or you have problems with your purchase or seller.
All banks are there to assist and change your Visa to MasterCard. Just ask.