Looking at the results suggests that the numbers are used sequentially,
So the first number in a sequence is AAA000000, then AAA000001, etc
After AAA999999, then next number is AAB000000, and so on
After AAZ999999, the next number will be ABA000000.
After AZZ999999, the next number will be BAA000000.
So the first character changes after 26 x 26 x 10 x 10 x 10 x 10 x 10 x 10 tickets have been sold. i.e.every 676 million tickets.
Eventually (after 676 million tickets beginning with Z have been sold) it goes back to A again.
So the first character will gradually advance through the alphabet as the weeks go by, but there's not a new letter every week. (i.e. if the last ticket sold in one week is ABC123456, the first ticket sold the following week will be ABC123457).
http://www.euro-millions.com/results.asp
Chris