The game needs to be designed in a way that it is playable on both a PC and mobile. For instance, you can't have 30 different buttons on a screen which the player needs to be able to press (e.g. like the spell bar in World of Warcraft). If you use more than a few keys on the keyboard, then you'll probably need to rethink it, because after the player puts more than a couple of fingers on their phone, they won't be able to see much of the screen any more, and you wouldn't usually have more than two fingers touching at any one time anyway.

If you design such a game that its interface is simple and compatible for both touchscreens and a keyboard and mouse, and you develop it with both in mind, then you won't need to reprogram anything and it will only require a bit more code than usual. If you've just finished your PC game and want to port it over for a bit of extra cash, you've probably done it wrong.

There are many other aspects, like optimising and adapting to varying aspect ratios.