That never happened. That info about America becoming commonwealths, the states being joined into groups was from a cancelled table top game that J.E Sawyer was working on. Its not canon.
The commonwealths are canon, in New Vegas, look at ED-E, it says "Great Midwest, llinois" on the lisense plate. It also says "Southwest, Nevada 2052" on the Bumper Swords.
The Great Midwest and Southwest are 2 of the 13 commonwealths.
In Fallout 3, the flag by the lunar lander has 12 stars in a circle, with one in the middle. This shows the 13 commonwealths were created sometime between 1945 and 1969.
Also, the idea of the commonwealths was briefly thought of in Fallout 1's development, but was never really expanded until the tabletop game Josh worked on.
http://fallout.wikia.com/wiki/United_States_of_America#Behind_the_scenes