To change the the rim size on a bike you should consider a 1.5"" drop in tyre width for every inch bigger you're making the tyre. As you increase the size of the tyre, the width of the tyre will increase too, so by reducing this it will make the tyre fit within the frame. Moving down in size is much easier and will allow you to fit wider tyres on the rims, increasing comfort and ridability. However, it's always worth checking out the clearance of the frame, and whether there is any adjustment needed in the brakes.