The accelerating digitalisation of financial services has transformed how retail banks architect, operate, and govern their information systems. Within emerging market banking environments, the transition from on-premises, monolithic estates to programmable, service-oriented cloud platforms has become a strategic imperative rather than a discretionary investment. This paper examines the practical experience of a South African retail bank undertaking a large-scale modernisation programme, and it distils enduring lessons for similarly situated institutions across the continent. Drawing on multi-disciplinary literature spanning enterprise architecture, cybersecurity engineering, data management, regulatory compliance, organisational change, and financial governance, the paper traces the full lifecycle of modernisation: strategic rationale, architectural pattern selection, legacy portfolio rationalisation, security and continuous compliance, data and analytics modernisation, workforce transformation, operational resilience, and cost stewardship. Particular emphasis is placed on how local regulatory instruments, including the Protection of Personal Information Act and supervisory guidance from the South African Reserve Bank, shape design choices relating to data residency, encryption, third-party risk, and audit traceability. The analysis contrasts the six canonical migration options—rehosting, replatforming, refactoring, repurchasing, retiring, and retaining—and clarifies the circumstances under which each is appropriate for core, channel, and analytical workloads typical of retail banks. The study finds that successful outcomes depend less on technology selection and more on the coherence of governance, the maturity of engineering practice, and the sophistication of change management. It concludes by articulating a sequenced roadmap and a set of evaluative heuristics that African retail banks may adapt in pursuit of more resilient, secure, and economically sustainable digital platforms.