OpenTofu is the direct answer, because it forked from Terraform at the point the licence changed and kept compatibility as the point of the project. For most teams this is a swap rather than a migration.
What you are replacing
Terraform is still the centre of the ecosystem and most providers, modules and documentation assume it. The reason people look for an alternative is the licence rather than the tool.
Configuration and state carry over, and the provider ecosystem is shared, so the change is small enough to be reversible. It exists specifically to keep infrastructure as code under an open licence, which is the concern that starts this search.
What it does not replace
Compatibility is a moving target rather than a guarantee. As the two diverge, newer Terraform features and vendor tooling that assumes Terraform by name become the friction, so pin versions and check the provider you depend on most.
Stay on Terraform when your platform team depends on the vendor's own tooling and the licence terms do not affect how you use it. This is a licence decision first and a technical one second.