You couldn't switch between air and ground battles seamlessly, for one thing. Noa Izumi's entire motivation would have to be altered in Patlabor. Zone of the Enders would just seem wrong to me. Ryusui's already gone over Gundam 00. The original Gundam would either lose its space battles or its ground battles depending on which direction you went with. Gundam Wing is also difficult to imagine, given the Gundam's all-terrain nature was a major point early in the series.
Macross might work, since the series is heavily steeped in the aircraft function of the variable fighter, though you'd still have to keep the mecha on the Zentraedi's side unless you completely change the size issue.
Given the severe lack of conquering any nation has done in this day and age, I think Code Geass required the Knightmare Frames as the reason for Britannia's military dominance, especially since it allowed a baseline for the arms race as the series progressed. At the very least, it's the most clear-cut technological upgrade that could be defined visually.
That's just off the top of my head right now.