Dimensions are usually hierarchical and they can change.

Tuple-versioning – Wikipedia, the free encyclopedia.
http://en.wikipedia.org/wiki/Tuple-versioning.

Slowly changing dimension …
Methods to handle situations where dimension data changes.
http://en.wikipedia.org/wiki/Slowly_changing_dimension

Type 0: Never update

Type 1: Update existing record

Type 2: Create new record with new surrogate key

Type 3: Update existing record but keep one previous history

Type 4: 2 tables with each dimension record previous versions stored in history table.

Type 6: New record with same surrogate key but different effective dates. Fact table will have natural key.