Is it technically possible to change the total amount of Bitcoin?
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.