Wondering if I have to replace the memory cards currently installed in my early 2011 13-inch MacBook Pro with two 8 GB memory cards or am I able to just ADD two 4 GB cards to equal what I need (8 total). Can't find an answer.

You can use either, if you are likely to further upgrade (to 16GB) in the future use 1 x 8GB chip.

The 2011 MBP has two memory module slots. Depending on how much memory you have installed, and if both slots are now occupied, you will have to add accordingly.

If you have one 4 GB module, add one 4 GB module to get 8 GB. If you have two 2 GB modules you will have to replace them with two 4 GB modules to get 8 GB.

Make sure you get quality memory modules as Macs are notoriously sensitive to their memory - see sources like OWC, http://www.macsales.com or Crucial, http://www.crucial.com