Topdressing and Reseeding a Weedy, Uneven Lawn

South Georgia lawn more weeds than grass, low areas pool water. Lawn guy recommends 1" topsoil overall, more in low spots, then reseed. Concern: will it kill existing grass? The expert says if you don't want the existing grass, kill everything and start over; topdressing plan is good. If you want to keep grass, need selective herbicide, which requires plant ID, chemical selection, careful application—often needs a specialist.