Well, that's exactly what Block 3 is, it's essentially a Block 2 with an upgraded FBW, so there's no technical reason why previous Block 1 and 2s can't be upgraded to Block 3 standard in terms of avionics and weapons. The chin hardpoint shouldn't be an issue either, nor should an AESA upgrade. What might not be possible is changing deeper internal structures to increase fuel capacity. The PAF has repeatedly stated that future changes would be added to previous blocks, and given the iterative changes and upgrades, I can't see why this isn't feasible, especially given the PAF's experience with rebuilding and upgrading other airframes, such as the Mirages.