Network functions virtualization (NFV) is an evolving field with great potentials, similarly is the software defined networks (SDNs) field, which is considered as a complement of the former. As the cloud services grow to dominate most of the web services, it becomes obvious to consider cloud-based centralized radio access networks (CRANs) as an excellent approach toward NFV. The three technologies have significant potential to improve the quality of service (QoS) while decreasing networks operating cost. Several new research proposals discussed how to deploy these technologies under new infrastructure, but only few investigated incremental transition from the current networks to the new deployments allowing a hybrid mix. The transition process faces three major issues: 1) which cell sites to be updated; 2) how much updates to be applied (i.e., change to fully virtualized or not); and 3) where the virtualized cells should be positioned. A common factor among these issues is that all are influenced by how much centralization is employed in the RAN. Therefore, this paper presents a new integer linear programming (ILP) model to allow decision makers (e.g., business managers and network architects) to investigate the impact of centralization level on decision criteria (e.g., required investment) by controlling the centralization level and its associated weight (reward). Our model shows how the investment should be positioned based on the level of centralization and the relationship between the costs of different resources. Additionally, we propose a time efficient heuristic algorithm to provide a reasonable suboptimal solution for the model under scenarios with a large number of network nodes.