I did this swap on a watch that I gave to a friend a couple of months ago. I ordered TWG's 21j Asian Auto Sub, and had an ETA laying around that fit perfectly. The only items I changed on the watch were the movement, the movement ring (got rid of the plastic one and used a metal one I had with the screws and clamps), the hands (appropriate 90/150/25 for ETA), and I changed out the bezel insert for a genuine. As for actual modifications that need to take place in order for this swap to work-
1) The dial feet need to be cut off, and the dial needs to be fixed to the movement spacer ring as the dial feet locations are completely different between the two movements.
2) The datewheel overlay from the 21j has to be swapped to the ETA (unless your ETA already has the appropriate overlay on it; in my case, I had to do the swap, as the ETA was new).
Here's what I started with...
21j Asian Auto is uncased, and here I've made the datewheel overlay swap.
Dial and hands installed, ready to go back into the case.
Finished up...
Final product.
Hope this helps!