You might be able to do this, but it may not be a good idea.

The goal with exhausting rising cooking effluents is to move them out in a straight direction, with as little loss as possible and without bends -- or doubling back down and out the floor.

If the structure of your house allows this, its probably better to add the hood above that you want and route new duct either up (out the roof) or horizontally (out an outside wall). This will likely be more effective than the old downdraft (especially for stockpots or taller pots), which you can easily close or block up.

Or revise the lower ducting to be a key part of a make-up air system.

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