Well, there are a number of lip change combinations possible, some favorable, some not. But that is a multi variable function of 'if this--then that- where there are many; 'if this' start points, displacement directions and amounts of displacements that could combine to make many 'then thats' as to final contour.
An advancement will bring the lip more into the FOREGROUND (closer to the eye of viewer) so it would not look any thinner. But if advancement starts really pushing on the lip too much, the soft tissue could push back and look thinner.