Not sure what you bonus you are referring to. There is no standing 10K MQM bonus for upgrading from Plat to Reserve and any initial sign-up offer would not apply in this case. Unless you specifically received a 10K MQM offer for the upgrade, I would not count on any such bonus.
This!! I have heard of no such upgrade offer. You are allowed to have multiple accounts, which means you can apply and get what ever sign up bonus is currently offered.
Have you met both of your platinum thresholds for boosts? Upgrading to reserve now, with no real offer would forever bar you from a sign up bonus with that product. It doesn't make sense.