user90053: Also, is it possible to change the reward cycle duration or the number of mining- reward coins?

user24974: Technically, yes, yes, and yes.  However, I'm not sure if we'd still call it "Bitcoin" at that point.
Any of these changes would require a hard fork, which means that the underlying rules of the system are changing in a non-backwards-compatible way.  It means that blocks/transactions from after the fork are not guaranteed to be considered valid to nodes running software for pre-fork.
So, to change Bitcoin like this in practice would require convincing the entire rest of the world to agree with the changes.  Anything less, and you will end up with two versions: the original and the one with your changes.  Which one gets the title "Bitcoin"?  I don't know.  That's a non-technical question, and a point of contention.
For an example of this in practice, see Bitcoin Cash.

90053: <p>Also, is it possible to change the reward cycle duration or the number of mining- reward coins?</p>
 24974: <p>Technically, yes, yes, and yes.  However, I'm not sure if we'd still call it "Bitcoin" at that point.</p>

<p>Any of these changes would require a hard fork, which means that the underlying rules of the system are changing in a non-backwards-compatible way.  It means that blocks/transactions from after the fork are not guaranteed to be considered valid to nodes running software for pre-fork.</p>

<p>So, to change Bitcoin like this in practice would require convincing the entire rest of the world to agree with the changes.  Anything less, and you will end up with two versions: the original and the one with your changes.  Which one gets the title "Bitcoin"?  I don't know.  That's a non-technical question, and a point of contention.</p>

<p>For an example of this in practice, see <a href="https://www.investopedia.com/tech/bitcoin-vs-bitcoin-cash-whats-difference/" rel="noreferrer">Bitcoin Cash</a>.</p>
