This was an "inversion" and the income tax treatment you describe is exactly in line with transactions of that sort.
It's "as if" you were paid in cash for the old stock and then immediately turned around and bought the new stock with the cash. If you still have the very thick document that explained the transaction to you look for a section titled "Material U.S. Federal Income Tax Consequences" or something similar and I'm sure that this methodology was explained there.
Tom Young
