I have found that the architect can safely play the role of Technical Project Manager without becoming stale or being detrimental to the success of the project. However, the Technical Product Manager role is something very different. Are you talking about guiding the project to completion or defining the product to the point where it can be built? I suppose it would be good to know what the new position entails.
A Product Manager isn't usually a highly trained, technical person. They have to be good at listening to business requirements and synthesizing them into something that would best be classified as a Product Requirements Document (PRD). Some places call this a General Specification Document (GSD). If you're using Agile/XP, the Product Manager may go as far as writing story cards and detailed stories that flow from them. They might write data flow diagrams but never technical details like schema or service and operation contracts.
I've worked with highly technical Product Managers in the past and it's rarely a good time for anyone involved. Getting a specification to just the right level of detail often requires that the Product Manager doesn't know how to "make the sausage" if you know what I mean.