Are those "slight improvements" really improvements at all?
Weren't those circled parts consistent with what we first say on J-20 s/n 2101?
The only slight possible difference might be that the dielectric grey fairings on the tailstings are slightly different to 2101 in terms of coverage (the overall geometry of the tailstings are the same as 2101), but even then the overall geometry of the tail sting is identical as 2101.
Comparing with this older image showing 2001 vs 2101 vs the latest 210x there are indeed a few differences. IMO however the most obvious one - IMO chaff & flare dispensers - were omitted !