Your original route registration probably looks something like this:
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new
{
controller = "Home",
action = "Index",
id = ""
} // Parameter defaults
);
The key is that your area has an AreaRegistration where you set up the routes for that area. Inside the area registration, it looks like this:
context.MapRoute(
"MyArea_default",
"MyArea/{controller}/{action}/{id}",
new { action = "Index", id = UrlParameter.Optional }
);
Notice how the first segment of the route that you've defined has "MyArea" (or call it whatever you want). This is how the area is differentiated from the default routes.
So, to conclude, you will not have to modify the routes you already have set up in your global.asax. You will set up routes for your area and they will be differentiated as shown above and conflicts will be avoided.