Another consideration is modding.
With the current engine, it's very easy to modify the data. Making, installing, and managing mods is all very simple compared to most other games. In moving to a new engine, you could lose this.
However, what your engine is currently lacking is some way of modifying code. A new engine may make it easier to implement some sort of scripting api, an often requested feature. This would open up the modding potential of your games.
I know the modding community for your games is rather small, but it is something you ought to consider. The change to a new engine could help the modding community grow, or diminish it further.