And that just made me realize something....
Being the thickness of the oxide layer IS voltage dependent, and it is that thickness which sets the capacitance, you might find that after a while, when the dielectric deteriorates to the thickness of the voltage you are operating it at, the capacitance will rise. Therefore, I would have to correct my above post and say that, yes, it will work fine, at least for a couple years. After that, I would anticipate an increase in capacitance value as that 50v thickness wears down to a 12v thickness.
Edit: but it probably won't hurt anything if it's just used as a stabilizing or decoupling cap. If it is used for a time constant type circuit (as in audio filters, NB's and ACG circuits), then you may have an issue later down the road. This is just me making what I think is a logical guess, but I honestly cannot tell you how much that value will change.