You can conver some of them into Research or Military Production by clicking the icons at the top of the colony management screen for each area (it focuses your production on that world on one of the three areas at a time). Other than that, unused production capacity is just wasted.

As far as the money issue goes, it doesn't get "converted" into cash, but the effect is the same: every point of production capacity, whether it's military, social, or research, costs 1bc/turn to actually produce something. But if you aren't producing anything of that type on a world, the game is aware of that and doesn't charge you for the idle factory/empty lab/whatever. So you will have a greater income per turn if you're not producing anything, since your expenses are lessened.

- Ash

As far as the money issue goes, it doesn't get "converted" into cash, but the effect is the same: every point of production capacity, whether it's military, social, or research, costs 1bc/turn to actually produce something. But if you aren't producing anything of that type on a world, the game is aware of that and doesn't charge you for the idle factory/empty lab/whatever

Well, sorry to contradict you, but with an empty queue, social production is wasted, contrary to what the manual states. You can easily test it: set your tax rate to 0, general spending to 100% and 100% on social. Note the number for you budget, your treasury, hit turn and see how much you treaury has decreased.

From this thread https://forums.galciv2.com/?ForumID=346&AID=105246 post #5

Social production is indeed "Wasted". This was a game balance decision. The manual is incorrect is all. It would be much harder to manage your economy if the net revenue was changing drastically every time you got a new technology that started upgrading your planetary improvements. We plan to eventually make it so that social production improves your approval rating. But it will need play testing.

So it is official: wasting social production when building nothing is a feature and not a bug.