Keep the letter, renew AMBASSADOR if you are so inclined, and see what comes. While Plat is no longer the status given with AMBASSADOR -- just ROYAL AMBASSADOR -- there have been lots of exceptions made, and given you have been Plat for several years, this may be a courtesy offering. We know lots of PlatPCs/AMBASSADORS who are getting courtesy ROYAL AMBASSADOR based on their overall business to the chain, and not just one the suspected IC stay requirements, so que sera sera.

If you end up getting GOLD, then fax your letter to the service centre with a complaint that you were promised PLAT, and they'll likely honour it if there is otherwise a glitch.