smaller wheels on big bike to reduce height
The first problem with changing the wheel size is surely the brakes. Assuming they're rim brakes, they won't be in the right place for a smaller wheel.

In any case, is it the distance to the ground that is an issue or the distance to the pedals? If it's the latter, then changing the wheel size won't help, however, you might get an extra 5 or 10 mm by using shorter cranks - if this is appropriate for your height.

I would imagine that it's probably better and not much more expensive to trade the whole bike in for one which fits you.