Another issue to consider is, in order to maintain rolling radius, or wheel diameter, the profile needs to change in direct ratio to the tyre width. Since the profile is given as a percentage of the width. e.g, a tyre with profile 50 (%) with a tread width of 200, would mean the profile equates to 100mm (half, or 50% of the width). So changing one and not the other will result in a change in wheel diameter/rolling radius.

Use a tyre size calculator to ease the headache:

http://www.discovery2.co.uk/tyresize.html