2-3 weeks ago I emailed devprograms about this, and 2 days ago received the following reply (slightly abbreviated to avoid upsetting anyone at Apple):
"Thank you for contacting the Apple Developer Connection regarding the iPhone Developer Program.
Please accept our apologies for the delay in responding to your request.
As per your request, we can arrange to switch your current enrollment from Individual to Company.
Please be aware that your access to the iPhone Developer Program Portal will be temporarily disabled while we conduct the migration from an Individual to a Company type
You will still be able to log in to the iPhone Dev Center and have access to all of the Registered iPhone Developer content on the iPhone Dev Center during the migration, but you will not have access to the iPhone Program Portal until the migration to Company type is complete.
Please also not that you may be requested to fax in business documents for your company as well.
If you would like us to migrate your enrollment to a company type then please let us know.
Best regards,
..."