From your pictures, the plant is already clearly root bound. A root bound plant will have soil volume replaced by roots, and will eventually be stunted by lack of water and nutrients. If you want to keep it in the same pot, it's simple. Just do root-pruning. That's where you trim off the roots at the periphery, circling the pot, and tease out the inner roots so they aren't as tangled. Then just repot in the same pot. Not sure if this strategy is a good one at the end of a growing season when the plant is entering dormancy, but if it's going to stay warm for another few months it ought to work.

Thanks Dan. I think the plant is close to dormant as it stopped growing and started yellowing and dropping leaves. I already pruned back the branches and leaves and top dressed. Do you think I can prune the roots in early spring when it starts to put out new growth, or should I do it now? Lows will stay above 55 F for the next 2-3 weeks.

Actually, on second thought, with trees and container florals, root pruning is usually done when they are dormant. So my guess is that you're good to go right now. Your plant may show signs of shock for the first week, but just keep it well watered and out of direct sun.