I can give you a solution, but i am not sure this is the beat one out.
<mx:Button label="Register" id="submit" click="checkUser();"/>
1. Create a instance of the MXML should be loaded like this in the checkuser()
var userform:UserForm = new UserForm();
var myCan:Canvas = new Canvas();
parentApplication.removeAllChildren();
myCan.addChild(userform);
parentApplication.addChild(myCan);
Here the UserForm is the MXML to be loaded after clicking the button "Register".
Let me know the result after your try.