For quite a long time PostgreSQL supports authentication via SCRAM-SHA-256 now, so migrating is long overdue. This small guide will show you how to do this.
The support for SCRAM-SHA-256 was introduced in version 10 of the PostgreSQL database and it is the default for freshly installed systems since version 13.
Since it is not only used for internal storage of password hashes on the database side but also for clients which have to hash their connection password in the correct way there likely was some impact when databases were upgraded. However in some cases clients were not ready and so sometimes the DBMS was configured to stick with md5 and most likely forgotten. ;-)