Cloud Computing is the dynamic provisioning of physical and virtual resources as services offering by providers, to optimize performance and utilization of their resources. Consumers contract Cloud services and negotiate service level agreements. Some consumers plan to migrate functionality of their legacy systems to Cloud Computing, to minimize investment on their own infrastructure and to obtain new software solutions that rapidly adapt to changes in the system environment. Therefore, the contribution of this work is to classify different types of migration of legacy systems to Cloud Computing, according to the characteristics of the applications and the deployment models of Cloud Computing. Thus, a workflow for functionality migration is proposed, based on the experience in system conversion projects and the analyzed characteristics of Cloud environments. Finally, some security risks are evaluated in the migration process and some recommendations are listed.