elaborating on Don Myshrall's comment above--No you do not need underlayment as it is already there under the old roofing. Good roofers when reroofing will typically but the top edge of the new shingle into the bottom edge of the old. As long as the existing old roof was laid-out evenly, this results in a nice clean tight reroof job. Building felt would only tear and crinkle up and muck up the job.
But, heh, if the manufacturer's specs require it, then you are stuck.