OK, looks like we finally have a complete export of the database, so I can proceed with the forum port / upgrade.

5) I know that we have at least a few duplicate email addresses, so I will clean up the DB before proceeding.

Yes, that is definitively necessary. I recommend to create a second backup of the current state after the check for doublettes for the case of a failing upgrade in the furter process steps.

Yes, will definitely do this.
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Our current version of MySQL is 5.6.51. The new instance MySQL version is 10.3.38.

MySQL 10.3.38 sounds like MariaDB. That has nothing to do with the PHP version and it should not be a problem.

Ah yes, it is MariaDB. Thanks for the note.
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I've had a heck of a time exporting the data from the existing instance / DB, as either the script or DB is timing out before the export is complete. The entries table has 500,000 records and only 300,000 records could be exported before the timeout. I've got a piecemeal solution we're going to attempt today that should work.

Wow, 500,000 entries? That's a lot.

Yes, 494,000 entries. It is a lot, but we’ve never purged any records to date. Performance has suffered dramatically recently, so one of the drivers for the upgrade was, well, to get our forum current and see if we still have performance issues. I do notice 388,000 records in the entries_cache table. Might that be a concern here, and are there any notes on the purpose of the entries_cache table anywhere?

And for clarity, there are maybe 500 (most recent) posts which are actively viewed. The reminder are all for historical research purposes.

Thanks again for all your help.