Just be careful with doing that. I have ran into cases where they have taken the center part of one bearing and the outer part of another bearing, and then put the standard number seal in them. So you go buy the bearing according to the part number on the bearing to find out that they have used an outer race that is .004 smaller than the standard bearings.
Bearing normally measure what they say they measure. I am getting 1.125 as 28.575 mm the 6206-2rs has an inside diameter of 30 mm or 1.181 as opposed to the 6206-18ddu which is 1.125 or 28.575 mm So there is an inside diameter that is special to the 18DDU bearing.