that's a good question, I had to rip out all my plants before putting in ours, and I didn't put the bed int he one spot that has a giant azela bush I love that I wasn't willing to loose.

Most plants won't take to being buried that deeply. Tree roots may be smothered, other plants may rot around the base. What is growing in this area now?

The short answer is yes. You cannot add 12" inches of additional soil to any planting area and expect the existing plants to survive - NO plants will tolerate being buried that deeply. They'll be smothered from lack of oxygen and/or visually obliterated by 12 inches of soil piled up around them. They will have to be removed and replanted after raising the bed and adding however much soil you wish.