Continue working with HP support through the direct channels such as email and phone. The HP forums are not actively monitored by HP support reps as this is a User to User communication tool. To sort out warranty issues you are going to have to work directly with HP.
Keep up the good fight. HP somehow has the impression that this printer has an expired warranty and you need to help them correct the error. Once the warranty has been verified im sure HP will shoot you out a replacement printer in no time just like they would for any other warranty situtation that is valid.
Honestly, there is something fishy going on with the OfficeMax that you purchased this printer from. They should not be selling you new printers with outdated warranties or service contracts in the first place. You have a reciept from them and in a worst case scenario should be able to press back at them for a refund should relations with HP fail. OfficeMax should be able to stand by the products that they put on thier shelves and if they are selling refurbished or previously owned equipment then they should be held accountable for it.