I've had 2 separate spinal fusion procedures done on my neck, first at the C5-C6 level, and 18 months later at the C7-T1 level. When they did the second surgery they removed the plate and screws from the first procedure because the bones were completely fused. I don't have any experience with a "cage" since my surgeries happened in 2017 and in late 2018, and they used a piece of donor bone, along with plates and screws.
If they recommend removing the cage and replacing it with a plate all the way down to the T1 level this may be your best option, but I'm not a doctor, just someone who's experienced something similar to what you're going through. Good luck.