Mohammad Daoud has some suggestions for Selecting a New Microsoft Dynamics GP Partner. I think that there can be some differences when selecting a partner for a new implementation versus a change for an established entity but I’ll let you read Mohammad’s piece and draw your own conclusions.
Mohammad R. Daoud: Selecting a New Microsoft Dynamics GP Partner
Archived post from 2012. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.