Yikes - that is one large cacti to have indoors. Yellowing of cacti often means too much water and not enough sunlight  when grown indoors. Ants on plants indoors may be a sign you have a nest someplace so treat for indoor ant infestation. To repot cacti be sure to use a potting soil made for cactus or make your own cacti potting soil by adding one third Perlite or sand to regular potting soil. This will ensure quick drainage so that the cacti won't rot. If possible move all cacti houseplants outdoors for the summer. Feel the soil by poking a finger down to second knuckle. Do not water if any moisture is felt in soil. Do not let cacti stand in water that collects after watering. Summer is coming so your indoor plants will soon be happier. Keep growing, Marianne Binetti