Your current tattoo has a solid black design and defined shapes, it can likely to be covered up with new design that will need to add dark tones and bold elements. Here are the Coverup Tattoo ideas that covers your current tattoo: 

No issue with knees? Good luck. Both of my knees are done. The left one got so swollen that i couldn't bend my leg. The black and grey on my right healed better, but it still hurt a lot. Knee and elbow ditches? Piece of piss.