What happens next?

How to grow tomatoes upside down
Fill your seed starter cell with moist potting soil.
Continue watering the tomato, keeping it warm, and providing it with plenty of sunlight as it grows. The plant should be about 6 inches (15 cm) before you transplant it to the upside-down planter.
This will ensure the plant and the root system are established enough to take root in the new location. Don't let the tomato grow any bigger, otherwise the roots may get damaged when it's transplanted.