This study uses three case studies to investigate how the installed base affects Electronic Health Records (EHR) implementation in European hospitals: i) transition from paper-based records to EHRs; ii) replacement of an existing EHR with a similar system; and iii) replacing existing EHR system with a radically different one. Using a meta-analysis approach, the study employs the theoretical framework of Information Infrastructure (II) to analyze user satisfaction and resistance. Results show that the existing infrastructure and time factor significantly impact EHR outcomes. Implementation strategies that build upon the current infrastructure and offer immediate user benefits yield higher satisfaction rates. The study highlights the importance of considering the installed base and adapting implementation strategies to maximize EHR system benefits.