Modify Alias Stability to have two subclauses, with two different applicable versions. The first clause is the existing text, while the second restricts reassignment.
The concrete changes are:
Applicable Version: Unicode 6.2+
Property aliases and property value aliases, once defined in the Unicode Character Database, will never be assigned to different properties or property values, respectively.
Property aliases are defined in the file PropertyAliases.txt in the Unicode Character Database. Property value aliases are defined in the file PropertyValueAliases.txt in the Unicode Character Database.
This policy means that aliases cannot be reassigned to different values. For example, in PropertyValueAliases.txt there are the following data lines establishing the relation between different aliases.
ea ; N ; Neutral
ea ; Na ; Narrow
The alias "N" cannot be moved from being an alias for "Neutral" and to being an alias for "Narrow". Thus a change such as the following is disallowed.
ea ; Ne ; Neutral
ea ; N ; Narrow ; Na