Using the Speed Factor system and rolling initiative every round... you don't get to change your action after you declare it.
So as was mentioned... before you roll initiative for the round, each PC and monster declares what they are doing-- moving to Creature X and attacking with a polearm; casting Fireball; firing a longbow; or whatever. Each attack is given a bonus or penalty to their initiative based upon what they have declared. Then, each PC and monster rolls a d20 for initiative, adds in their DEX mod bonus, and then adds or subtacts the speed factor for the weapon/spell they were using. Thus, the initiative order gets set, and everyone does their action that they declared at the top.
The important thing to remember however, is that once you declare what you are doing before you roll initiaitve... you don't get to change it after you see where you go in the order or what the other creatures do. So if you declare you are casting a fireball but roll really low... and the other monsters go before you and declare they are running up to you to attack with their weapons... you don't get to change your action to cancel the fireball and do something else (like back away). You're stuck waiting for the fireball to go off, eat the attacks of the monsters, and hope that your spell doesn't get fizzled.
It can be a very harsh system, which is why it's completely optional. Some people like this idea that everyone decides what they do simultaneously and then let the chips fall where they may.