if at some instant of time in the future we needed to manipulate the supply of Bitcoin then we need to rewrite the Bitcoin protocol rules, a BIP will be proposed
First of all: you correctly stated that for such change we both need a
'code change' as well as a
'consensus change'. If one of the 2 is not given, the main Bitcoin blockchain won't have more than 21M Bitcoin.
In theory, you could change any parameter in the code, recompile it and run it on your machine.
For instance,
here, you could modify the halving interval and the software could accept 6.25BTC blocks forever.
However, since the rest of the network won't relay such blocks after the next halving (requiring miners to 'send' themselves only 3.125BTC per block), there will automatically be a chain split and you'll be on your own chain with nobody else on it. That's why you also need full nodes to follow.
You may also want to read this whole answer:
Consensus changes to Bitcoin are made by the entire ecosystem transitioning to new rules that they choose, by running node software that enforces these rules.
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