Please note that the AK prefix on the 50-birr note (US$4.40) represents an overlap in serial prefixes with the 2006 date, which is available in AK and in AL. Normally prefixes in Ethiopian notes continue the progression (the only other example of this that I know of is 100-birr Pick 34 where the serial prefixes for varieties a and b overlap).
For the current designs, replacement notes (AZ) have been confirmed for 100-birr notes (US$8.80) dated 2000 and 2004. If anyone has information regarding other replacements, please send an email with scans to confirm.
Courtesy of Chris Hall.