400 milions are mined, who want to start mining a 1.2b coins ? 1/3 is mined, what community will have a coin like this ?
This is the hurdle that this or any incumbent coin would face in this position. You see the backlash of the current coin supporters, however losing them and/or causing a second fork of the coin would be detrimental to the coin in name. As has been said, you'd just be better off starting a whole brand new coin. Having existing people that support the coin is necessary - without them you'd be fighting an even bigger uphill battle, and again, you'd just be better off abandoning the coin all together and starting from scratch.
The only major thing going for it in this scenario is that it's not a private premine. That helps a lot, at least in my eyes. I would think it would in everyone's as well.
To answer your follow up question, ideally I wouldn't put the coin extension past 2 billion coins. That puts the coin at 20% mined rather than 33%, but again, the higher you put that number the higher the perceived dilution of the current shares that people hold and hence the more resistance to any such plan you'll get from current supporters.
It's a balancing act that gets to be played. You can't make everybody happy here which is why I said EVERYBODY will need to compromise.