There are two ways that you can go about this, you can use a switch, or you can use a variable.
A Switch has an on-off state, you set it to on and then you set one of the pages in the event manager to check the state of the switch. If the switch is in the proper position then you can make the person say something else.
Variables can store different values. But the way that you would go about it is the same as a switch would be. You store a value with the variable, check it with one of your pages and then display different informations based on what comes up.
Alternatively, IIRC, you can also put a check for the state of a switch or the value of a variable in the event-logic-flow-window-thing (I don't remember what it's called). That should let you do the same thing.