symbol change history

["cwest" <cwest@xxxxxxxxxxxx>]

Does anyone know of a source of historical symbol changes? I've
discovered that probably all data vendors change the symbols (for
stocks) when the exchanges do, but what about the history! For example
if I have downloaded 20 years of data for a given symbol, and it's
symbol subsequently changes I'm wondering how I'd reconcile the new
prices etc. to the old symbol.

Thanks in advance
Colin West