You have to have upgraded to a new phone as of the date the new 3 tier tab structure came into place, which was only a month or two ago. If you didn't get a new phone within the last two months then no your not eligible.
You can upgrade at any time however with what remaining tab you have available on the small tab or move up to one of the larger tabs if you are eligible. To be eligible for a medium tab your plan must be $30 or more before taxes, overages or add On's. If it's not you can switch have to a new plan of $30 or more and then be eligible To be eligible for a large tab you have to choose a large tab plan being offered right now or at time of the upgrade to a new phone.