A WebmasterWorld thread asks why can't we update our tax information at the Yahoo! Publisher Network portal?
Well, the message on that page currently reads;
To prevent accidental modification, the Tax Information page has been locked from modification. If you need to change your business status from a sole proprietorship to a corporation, please contact Customer Solutions. Please note that a transfer of account ownership is not supported.
Seems to me, Yahoo! is preparing the tax work for their publishers and they wanted to freeze things as is for a bit of time.