There is a need to upgrade legacy system in industry to conform with safety norms and regulations defined by recent standards. The great investment for the development of these systems is the main reason for the industry to look for approaches to upgrade legacy systems instead of adopting a redevelopment of the whole system. In this paper, we describe an approach to upgrade legacy industrial applications based on the IEC61131 function block model without the need to redesign the whole application. The approach that integrates the 3+1 SysML-view model with safety engineering is adopted and is tailored to the needs of upgrading legacy applications. Challenges are identified and solutions are proposed towards the definition of the whole development process including the verification of the so generated safety application. A laboratory system is used as a case study in this paper to demonstrate the applicability of the proposed approach.