The transition from one ERP system to another is a complex and demanding process for every enterprise, associated with a numerous risks and unplanned expenses. This process is commonly ending and new information system is becoming operational, in the beginning of new business year. Practice shows that many of these projects are ineffective and ends in disputes. The subject of this paper is a new methodology, created as an attempt to answer one of the key issues that are critical to the success of this type of projects, and that is the successful migration of business data. The methodology presents migration from the existing to the new ERP solution: during the business year, on transparent way and in real time. Possible advantages of the methodology are numerous and they will be further systematically described in this paper. Developed methodology was successfully applied to the medium size enterprise in the complex business project that successfully ended, thanks to its usage in the planned time, scope and budget, on the satisfaction of all involved parties.