It's better to think of it as a hard forked upgrade. Normal BTC won't exist in any meaningful manner after this fork. Just like other coins hardfork all the tim

It's better to think of it as a hard forked upgrade. Normal BTC won't exist in any meaningful manner after this fork. Just like other coins hardfork all the time to upgrade. The previous chain could still exist if you wanted to run it, but no one does. Losing anywhere near 90% hashrate means that chain of Bitcoin has no point in being called Bitcoin anymore.