The Enterprise System (ES) forms the core of e-Business, providing support for operations, and analytical information for real-time decision-making. A well-functioning ES can enable organizational success and thereby create economic and social value. Once installed, upgrading an ES is important for improving performance, automating business processes, and reducing maintenance efforts. But ES upgrades also cause business disruption and downtime. As a result, many organisations cannot gauge when to upgrade ES. This paper explains the network of factors that influence the timing of and decision to upgrade ES and answers, why and when do organisations decide to upgrade their ES? Literature on upgrades, although scarce, presents opposing views on the timing of upgrades. Moreover, studies investigating this also appear to be dominated by studies from high-income countries where these systems are developed. Other countries potentially face different challenges. Hence, this phenomenon was investigated in South Africa, a middle-income country, through three qualitative case studies. The findings suggest that upgrade decisions are tightly controlled by executive committee approval and are aligned to the organisation’s strategy. In addition, the business calendar and risk management play a vital role in the timing of and decision to upgrade ES. The paper presents an explanatory model of how factors impact one another. The model should be useful to organisations wanting to upgrade their ES and extends the current ES upgrade literature.