1. They have China's support
2. They have nuclear weapons
However, the filtration the the US State Department's cables by Wikileaks has revealed some very interesting information regarding China's opinion of North Korea. Simply put, they're tired of them because they've outlived their usefulness as allies. China's development is at a stage where economic issues are considered more important than political ones. If the choice is between increased trade with South Korea and Japan or ideological support from a pariah state (whose ideology, by the way, is so different nowadays from China's as to be the complete opposite in economic and social issues), then there's really no choice at all because there's only one real option. The issue then is coming up with a way to bring about regime change without risking nuclear war.
The real force in North Korea isn't Kim Jong Il but the military. He's still in power because it's easier to rally popular support for a demi-god "chosen one" than for secular armed forces. Any proposed solution will have to consider this fact as pivotal. Then the real question is how to get military leaders to relinquish their support for Kim. The answer: power and money. They control both right now, but they are eroding rapidly as North Korea's economic fortunes go from bad to worse. They have one card to play, and that's the nuclear deterrent, but they know that if they use it they'll have lost all their bargaining power.
South Korea is in a position to bribe military leaders, providing financial security and probably even political appointments in the new regime. They could probably even get Japan to pitch in, as the monetary cost would be a trifle compared to the cost of war for these nations. China would quietly give their nod of approval, as their preponderant interest has to be the stability and economic development of the region. The costs have started to outweigh the benefits for China of having North Korea as their protege, so things should change sooner rather than later. US interests are aligned with that of South Korea and Japan, so they would back a unified proposal from these two countries.
The final consideration concerns the ultimate fate of North Korea. Will it be absorbed by South Korea, China, or remain an independent country? What will Russia's position be?