Patch 3.2.0 (2009-08-04): Gnomish Engineers and Goblin Engineers can now switch profession specializations for a fee.
Details[1]
* Read the book Soothsaying for Dummies which is on a table in Neutral [45] Narain Soothfancy's house. He lives in the northern part of Steamwheedle Port, on the northern coast of Tanaris (western continent of Kalimdor, Azeroth). Pay the 150g fee to unlearn your current specialization.
* To learn another (or the same) specialization, two different techniques seem to be needed, for different players. One is to read the above book a second time and you will be offered a choice. The second is to do the quest sequence (beginning in Ratchet) for the specialization you desire.
* Note: as of Patch 3.1, [Schematic: Lil' Smoky] and [Schematic: Pet Bombling] are no longer membership-renewal rewards. Instead they are drops from Gnomeregan mobs. These schematics are not specialized; all engineers (with suitable skill level) can learn both!