Ethereum's last hard fork, Constantinople, and the current ongoing Istanbul hard fork all bring in an important topic that wasn't previously strongly considered -- backward compatibility. In this presentation, we will explain why this is an issue, and why when reviewing EIPs for inclusion, only talking about soundness of the EIP is not enough. We will explore techniques that are being proposed to solve this issue -- most importantly, account versioning, and how it enables EIPs being included hassle-free, and also allow us to drastically change the VM in the future. The presentation will conclude with current challenges we are still facing regarding backward compatibility, and if time permits, traits that we can use when reviewing an EIP to understand whether it requires account versioning or not.