Hi there!
My 8 month old Flame Tree's lowest four or so branches don't seem to be doing so well?
The leaves are turning yellow and falling off. 
I haven't changed anything in its routine. I thought perhaps I had overwatered it so I changed the soil. It seemed to do better for about a day but then went back to yellow and wilting. Will do research, and I will update with pictures when I get home.
Ideas?

Flame tree is a decidious tree. If you are in the nortern hemisphere, now is authum and leaf fall is part of their natural cycle. So no issue at all.

Repotting at the wrong time of the year however can prove to be an issue, but you will not know until spring.

"Has not changed anything in its care for 8 months" may also be an issue. Adapting the care continously to season and phase in development is part of the hobby.