"a REINDEX operation on an index pg_upgrade'd from a previous release could potentially fail."

According to https://www.postgresql.org/docs/release/12.0/
"
In new btree indexes, the maximum index entry length is reduced by eight
bytes, to improve handling of duplicate entries (Peter Geoghegan) §

This means that a REINDEX operation on an index pg_upgrade'd from a
previous release could potentially fail.
"

Would a REINDEX CONCURRENTLY mitigate that risk, since
https://www.postgresql.org/docs/14/sql-reindex.html#SQL-REINDEX-CONCURRENTLY
says "6. The old indexes are dropped"?

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