If you call in using the N900, they will most likely tell you "oh, we're sorry, but you've got a smart phone, so you have to use the smart phone data package". To get around this, either buy a dumb phone or borrow one from a friend, put your sim in it, and call up and ask to switch. If you tell them "oh, I'm using this dumb phone now" and you're not on a contract, they will almost certainly let you switch. Even if you are on a contract, don't be disuaded.

My story: I subsidised my N900 purchase by buying a 2-year contract and HTC TP2 with $20/mo data and $30 voice shortly before the switch the EM+ happened. That meant I was only getting 300 voice minutes. After I got the phone, I bought a dumb phone, and used it exclusively, selling the HTC TP2 on ebay. While still using the dumb phone, having not used the N900 yet, I called and asked if I could get EM+. They told me that although I had to pay a $30 transfer fee, not only could I get the EM+ plan, but that would also mean I would be let off of my contract and I would be able to buy the dumb phone plan for $10 a month, instead of the $20 (even though part of my 2 year contract required I have a smartphone data plan). Anyways, the moral is "ask and you shall receive". Hope this helps.