When the main series will end; A will be added to differentiate between the main and new series.
As a very simple example, just for the sake of explanation; for LEI-15-0001 to LEI-15-9999, 9999 vehicles can get registered. For the 10,000 one, the series will move to LEI-15-A-0001 (if LEJ series does not get introduced, i.e. LEJ-15-0001).