The prebuild is not an exploit, but it should still be removed. You should not be able to switch production without at least losing some of your accumulation. This would make wonders much more difficult, fair, and would make you think twice about setting those cities to produce one - you could very well lose your hard work if you aren't careful.
How often do you see a brand new technology come into use immediately after it is researched, let alone before it is researched? The only times I can think of is when the technology is ordered into being, like military technology. To be able to start building something, like a granary, before you research pottery, is absurd, even when you think of production as an abstract accumulation of work - how could you start accumulating those shields before you even knew what you were going to finish?