I had single boas and was looking to upgrade this fall. I purchased some double boas but after wearing them around the house I had to take them back. The wires on one of the zones ran right along my ankle creating a nasty hot spot that I didn't want to deal with. I ended up going with burtons that have the dual zone speed lacing system and am very happy with my purchase.
In regards to the question when to upgrade... I usually replace my boots when they get too soft and they can't offer the response I need.