Answer: It's not unusual for the oldest leaves (those at the bottom of the plant) to turn yellow and die. The plant is simply putting more of its energy into producing new leaves and developing the fruit. Cherry tomato plants are relatively small so as long as the roots have enough room you shouldn't have to transplant them. They should be happy all season long in a pot about the size of a one-gallon nursery pot. Check the bottoms of the pots; if roots are escaping from the drainage holes, you should repot. If not, they should be okay. When you water, immerse the pots in a larger container of water and allow to sit for 5-10 minutes, then remove and allow to drain before setting them back into the terra cotta pots. Watering in this way will thoroughly moisten the soil, driving out any air pockets around the roots. You shouldn't have to water again until the top of the potting soil is dry.
Best wishes with your tomatoes!
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