I work in the IT side of the house. We support our Police Department and the Intergraph applications. When we first took over IT support for our Police Departmemt and were facing a major CAD upgrade, we hired Gartner to help us implement an IT Governance model. This was really helpful in establishing the roles and responsibilities and communication plans between IT, Police and the vendor. Our upgrade project was supported by Police operational staff, IT technical and project management staff and project management staff and support from Intergraph. In my opnion you need a team like this to be successful. Having a very good vendor project manager and implementer is key. We were lucky to have Tami Renton as our Intergraph Project Manager. She assembled an great implementation team. An outside consultant could not do this. When we had issues Tami was able to engage the right resources and the right time. She could connect to dots between the various modules and functionality. I took on the PM role for the County. This allowed the Police Department staff to focus on configuration changes, testing and training. It took the whole "village" to get this done and we were successful.
So in my opinion, I would never consider and upgrade without enlisting the vendor. You also need internal project management skills. You may be able to out source that capability, but the consultant would need to know your organization and skill sets. You need an engaged business partner. We had that with our Police Department. Lastly, the Gartner IT Governance model helped us communicate and effecitvely manage issues and support with our Police Department.