lexus-is2

Just had my 140k service done at the dealership and they mentioned that the transmission output seal is showing signs of dampness. I am told that there's not a leak of such but the dampness indicates a leak may occur very soon. Apparently cost of the seal is very cheap but labour is expensive. Anyone know how complex the job is and if there is any kind of DIY or should i get the dealerships to do it? What does changing the seal involve?