If you want to open a base saved with an old version of EDT, you may have to save your base with an intermediate version. All old versions of EDT can be downloaded from the Index Education’s Internet site, from the page EDT > Downloads > Old versions.
Directly open the base with EDT 2021.
Install the evaluation version EDT 2018 to save the base before opening with EDT 2021.
Install the evaluation version EDT 2015 to save the base. Then renew the operation with EDT 2018 version before opening with EDT 2021.