I don't think there will be a big difference between the two in terms of avionics. And the new engine will be a drop fit.
The avionics being designed for AMCA is next gen, it will be current for its time. Even the stuff going into current projects like LCA Mk2 and MKI MLU will remain relevant into the 2040s, never mind AMCA. It's basically a 5.5th gen aircraft, with airframe being the main limitation when compared to 6th gen. LCA Mk2's avionics will be more or less similar to what's on the Rafale and F-35 today.
Avionics is one area where we have done an incredible job of catching up to the West.