The need to upgrade the performance of existing building façade systems provides many opportunities for innovation and the creative reuse of existing building components. As many mid-20th century building enclosure systems age, they exhibit performance deficiencies, both in the form of deteriorating performance and in lagging performance as weighed against newer technologies. The installation of replacement exterior envelope systems on existing facilities provides many challenges, especially in fully occupied buildings. Over-cladding of existing metal and glass curtainwall and skylight systems, wherein portions of the pre-existing systems are integrated into the new construction, provides significant opportunities to resolve construction phase issues while providing improved functionality to the completed building envelope. This paper will address two case studies in which contemporary aluminum and glass elements were combined with pre-existing mid-1900s aluminum framed cladding elements to provide two wholly new enclosure systems with significantly improved performance characteristics. The case studies deal with two very different sets of project objectives, site constraints and project solutions, but share common approaches and eventual solutions. The paper presents the problems posed by the pre-existing systems, the site investigations conducted to confirm existing conditions and site restrictions, reasons for considering the specific replacement cladding approaches, specific project constraints, the approaches used in development of the new over-clad solutions, and considerations in the design, development, and construction phase delivery of these projects. Applicability of these approaches to other types of contemporary building enclosure types is explored as well.