- pg_upgrade to v14 on the current operating system - use replication, than switchover to move to a current operating system on a different machine - REINDEX CONCURRENTLY all indexes on string expressions
You could get data corruption and bad query results between the second and the third steps, so keep that interval short.
We did something like this, with the addition of a step where we used a new-OS replica to run amcheck's bt_index_check() over all of the btree indexes to find those actually corrupted by the libc upgrade in practice with our data. It was a small fraction of them, and we were able to fit an offline reindex of those btrees and all texty non-btree indexes into an acceptable downtime window, with REINDEX CONCURRENTLY of everything else as a lower priority after the upgrade.