Most cars as of the last many years have to have the dash pulled out to change a heater core or A/C evaporator...... unless you want a hack job, literally and figuratively.
I've done a few heater cores, they usually aren't fun. And often times you can ALMOST get pcs B out without removing pcs X, but then to remove that you have to take out pcs Y... you get the idea lol
Its good they're replacing the carpet, that stuff stinks for a long time especially with the heat turned on and floor vents selected.