[frankenst1]

10%, 50%, 80% doesn't matter. Any node who wants to change Bitcoin's code and rules is free to do so.

It just creates an additional coin and it already has happened dozen of times (Bitcoin Gold, Bitcoin SV, Bitcoin ABC, Bitcoin Cash, ...). Some of them claim to be "the real Bitcoin" but market consensus seems to disagree: All of them are either dead or valued at <1/100 of Bitcoin.

As long as you run your own node you define for yourself what Bitcoin is.