Featured BREAKING: Taiwan signs deal to buy 66 F-16 jets from US defence giant Lockheed Martin

[A.P. Richelieu]

F-21 is even more capable than the already very impressive F-16 Block 70/72 variant with superior HUD and payload capacity, but offered to India "only." Additional amendments include retractable refueling probe and aft decoy system.

Not much else is known.

You do not need to rename it to F-21 for this.
That would be an F-16 Block 80/82.
I can only see that LM is doing this to get out of contractual liabilities blocking India from producing large parts in India.