Append-only

Another property that we encounter is that blockchain is append-only, which means that data can only be added to the blockchain in time-ordered sequential order. This property implies that once data is added to the blockchain, it is almost impossible to change that data and can be considered practically immutable. Nonetheless, it can be changed in rare scenarios wherein collusion against the blockchain network succeeds in gaining more than 51 percent of the power. There may be some legitimate reasons to change data in the blockchain once it has been added, such as the right to be forgotten or right to erasure (also defined in General Data Protection (GDPR) ruling, https://gdpr-info.eu/art-17-gdpr/).

However, those are individual cases that need to be handled separately and that require an elegant technical solution. For all practical purposes, blockchain is indeed immutable and cannot be changed.