animating addClass/removeClass with jQuery

You could use jquery ui's switchClass, Heres an example:

$( "selector" ).switchClass( "oldClass", "newClass", 1000, "easeInOutQuad" );

Or see this jsfiddle.

Another solution (but it requires jQueryUI as pointed out by Richard Neil Ilagan in comments) :-

addClass, removeClass and toggleClass also accepts a second argument; the time duration to go from one state to the other.

$(this).addClass('abc',1000);

See jsfiddle:- http://jsfiddle.net/6hvZT/1/

Since you are not worried about IE, why not just use css transitions to provide the animation and jQuery to change the classes. Live example: http://jsfiddle.net/tw16/JfK6N/

#someDiv{
    -webkit-transition: all 0.5s ease;
    -moz-transition: all 0.5s ease;
    -o-transition: all 0.5s ease;
    transition: all 0.5s ease;
}

You just need the jQuery UI effects-core (13KB), to enable the duration of the adding (just like Omar Tariq it pointed out)