Based on what I am used to hear, I would have first said that it is not possible to change the total supply.

But after thinking a bit about it, two things came to my mind, and at the same time, the answer to your question.

1) Total supply is just a parameter, so according to me (as I said, I have never thought about this precise issue before, nor heard anyone talking about it!), the sole people who would have the power to change it are the miners, and no one else.
That is for the your answer.

2) But now let's go a bit further.
On the other hand, I am pretty sure this is a situation that will never happen. Just see how simpler improvements led to great debates!
So the only way for the total supply to be realistically increased, is through a hard fork.

How would it be possible for the total supply to be increased: through a miner vote.
Will it ever happen on the main chain? No, that is sure at 99.99%.