Only once - I threw an EMG P-Bass pickup in my 1979-80 Yamaha bass. Later, I had a tech add an EMG J-pup to the same bass. They remain there to this day... Since then, I've tended to buy instruments that I like the sound of in the first place, so swapping pickups hasn't really been a priority. Maybe it's my short memory or shorter attention span, but I'm always afraid I'll forget what the original pups sound like. Also, I have so much EQ available through amps and pedals that I can squeeze out just about any tone I need.