Mainly here I'm reacting to Maxwell saying
txid change couldn't be deployed as a hardfork at all, because that quote very publicly being used on reddit to defend "segwit as a softfork".
Would it change how transaction IDs were computed, like elements alpha did? Doing so is conceptually simpler and might save 20 lines of code in the implementation... But it's undeployable: even as a hardfork-- it would break all software, web wallets, thin wallets, lite wallets, hardware wallets, block explorers-- it would break them completely, along with all presigned nlocktime transactions, all transactions in flight.
So that's basically FUD?
No. What he was responding to was th original idea of changing txid calculation entirely too something new. This idea (the second option) is instead introducing a new txid calculation method which will work alongside the original txid calculation algorithm.
Additionally, on further thought, it will still require twice the lookup tables. There needs to be one for the transactions that version 1 txs can spend from and one for the version 2 txs. Version 2 txs still need to be able to reference the txid of a version 1 tx to spend from it or that ntxid for the version 1 tx also needs to be stored somewhere so it will increase the lookup table sizes.