Is it necessary to make a full backup after pg_upgrade with Postgresql?

pg_upgrade allows one to quickly upgrade big Postgresql databases, by taking advantage of the fact that the raw data does not need modification, it is only the configuration and metadata. However, I’m wondering how to handle the situation where the database was originally backed up using a full base backup, and from then on using archiving of WAL files. But how does that work with respect to upgrades? I tried to test it, so I configured postgres.conf of the new version to also WAL archive (to a different folder) and then upgraded using pg_upgrade. Both clusters produced some WAL files, however, it doesn’t seem to be possible to restore the database by using the original base backup combined with the WAL files (the new version refuses to start on such a directory, and the old version starts but only replays WAL files up to the point of the upgrade). So I’m wondering what the official procedure is regarding backup after using pg_upgrade. Does one have to make a fresh base backup or is there a way to only backup the changes made by the upgrade (WAL files or otherwise)? I can see there’s a procedure for upgrading standby servers but that is not the same scenario (although related).