I don't really think of it as upgrading, I think of it as gradually tuning a bike to fit me. The "perfect" bike may be out there, but I'll never have the patience, time, money or insight to hunt it down. My 4 bikes - 3 in use - have all come into my hands in sort of roundabout ways, only one was bought new, and I spend my time and money gradually tweaking things here and there. I've bought them or acquired them based on the frame fitting reasonably well and the price being reasonable, then I swap parts around or buy new ones as old ones wear out, and am gradually tweaking 3 bikes to be fairly specific for 3 types of use: 1 winter bike/commuter bike (durable, low-maintenance parts, comfortable), 1 mtb, 1 training/road bike (lightweight, good wheels).
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett