industrial to space stage

I'm pretty sure all you have to do is achieve space flight. If you look at the Spore 2005 demo with the space stage, Will's flying around in a UFO while the Star Wars-ey guys are still rolling around in tanks. So no unification required.

There isn't really much of a transition between late industrial and early space.

For example, the early space economy would just be one facet of the planetside global economy -- various nations may have assets up in space, whether those assets were sent up or were things already in space that were laid claim to, and they can do economic things with those assets, like selling material off an asteroid, or renting out research space at a space station.

As the game progresses and various nations (probably driven by the player a lot, because players are much more likely to be more excited about space than in-game political realities would dictate for the AI) focus more on space, especially space infrastructure, the game systems (political, economic, etc) would naturally transition to dealing more and more with space.

I'm confident that there wouldn't have to be too many drastic changes in the underlying systems behind the game. Sure, you'd need to move units around using orbital mechanics instead of over the planet's surface, and sure, the concept of what is valuable will change a lot as you discover entire asteroids full of platinum etc, but these changes don't necessitate a sharp transition.

So yeah, from a coding and gameplay standpoint, it probably won't be overcomplicated. It will be complicated, but it won't get much more complicated than playing planetside.