What happens if I add a mismatched RAM stick to my existing 2x8GB kit?

It will work, but you lose full dual-channel. Your motherboard switches to “flex mode”: the first 16GB runs dual-channel, the remaining 8GB runs single-channel. The performance hit is small — about 3-5% in CPU-heavy tasks — and you probably won’t notice it unless benchmarking. To keep full dual-channel, sell your old kit and buy a 2x16GB set.