## 1 Introduction  
Cities are racing against time to clean the air, cut emissions, and electrify their transport systems. Diesel buses, once the workhorses of public mobility, have become symbols of pollution and outdated technology. Yet, replacing the entire fleet with new electric models is neither fast nor affordable. This creates a pressing need for transitional solutions that bridge the gap between current limitations and long-term sustainability goals. A promising alternative emerges: what if humanity could transform the old into new by electriying diesel buses already on the road? Buses are not just a mode of transport--they are the circulatory system of modern cities, influencing air quality, energy consumption, and public health. To appreciate the scale of this transition, it is essential to understand the composition of the current bus fleet. According to the Cooperatieve Rabobank U.A. research and ACEA (European Automobile Manufacturers Association) statistics, in 2023, there were around 750,000 buses in total, with 535,000 designated for intercity routes and 215,000 serving urban areas, making buses a crucial component of the transportation infrastructure. Most of these buses are diesel-powered, but the adoption of electric ones has been on the rise. The momentum is shifting, albeit gradually, as policy, innovation, and public awareness converge. This trend is particularly evident in the fleet segment utilising alternative fuels, where the number of electric models increased by 11,000 last year, nearly doubling the previous count. Only 2% (18,000) of the entire bus fleet consists of battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hydrogen-powered vehicles (H2). This underlines a critical gap between climate policy goals and the current technical reality. As predicted, electric city buses have surpassed diesel in new registrations within the EU (European Union), achieving a 41% share of all new city models in 2023. This marks a significant milestone, especially considering that BEVs accounted for only about 10% of new city bus registrations. Such a shift signals a transformative moment for urban transport, where electric models are no longer niche but a growing norm. The trend is evident--the EURO 7 will be applied to cars and vans from July 2030, and buses and trucks (M\({}_{2}\), M\({}_{3}\), and N\({}_{2}\), N\({}_{3}\) categories according to UNECE classification) in the next 12 months. The tightening of emission standards will likely serve as a catalyst, accelerating the need for practical electrification strategies, including the conversion of existing fleets. The observed sharp contrast between new registrations and the legacy fleet raises an urgent question: What will happen to the existing fleet of around 215,000 diesel buses mentioned earlier, the engines of which will not be able to meet the limits of NOx (nitrogen oxides) and other emissions? Meeting the tighter NOx limits requires advanced and expensive after-treatment systems, such as selective catalytic reduction (SCR) and more effective diesel particulate filters (DPFs), which add to the complexity and cost of diesel vehicles (according to the aforementioned AECC association). These technologies, while effective, push the boundaries of economic feasibility, especially for older buses nearing the end of their life cycles. As a result, it is economically more profitable to convert diesel buses to electric ones. Several additional UNECE (United Nations Economic Commission for Europe) certification requirements are imposed in this case. This introduces a new layer of complexity: transforming legacy vehicles is not just a technical challenge, but a regulatory one. Among the key hurdles in diesel-to-electric conversion are the international safety certifications, initially designed for factory-built electric vehicles, not retrofits.