I'm a fairly new customer of BT Broadband though I've yet to experience any speed problems that haven't been solved by resetting the hub. A friend of mine did have some problems with their connection speed but
that turned out to be because the extension lead they were using was
too long!
If speeds really are terrible for you, it might be worthwhile putting a call out to BT or even checking out their Desktop Help.
I can't imagine BT not letting you UPGRADE your package to the Business option because of the contract. Hopefully there would be no issue as you would staying with BT!