So, about those Alchemy masteries...

[Xanthix]

Incidentally is it possible to change specialisation? I remember reading that Leatherworkers and Blacksmiths could change their pre-TBC specialisations. Does the same hold true for Alchemy?

You cannot change specs unless you drop and re-level alchemy. This may be because unlike specs of other tradeskills, when you drop a spec you do not lose the use of any recipes/items, so they wanted some other way of upping the "cost" of a respec.