First they told me I couldn't upgrade my existing account - so I applied for and got a new plat card. Then they called to say I could, so I upgraded my existing MP Visa card. The perfect test case I guess.

Results:

2 envelopes on the same day - both with $25 and 1,000 mile upgrades, 1 with companion cert. No way of knowing which came with which card - but I suspect the companion cert came with the new account.