Posting this to help others in the same situation.  The client system had a 3rd party modification to their SQL databases (by FYISoft / Renovo), adding table level SQL triggers that became broken with the migration (since the new SQL instance didn't have the 3rd party database that the triggers were designed to write to).

Error details below.

Find the tables with triggers, then disable.  (This mystery was solved with the help of Sage support).

Scenario:

Migration for upgrading to v2023 Premium.  (Source system is v2019 Premium).
Migration completes fine.
During company conversion, there is an error 15.
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The following conversion returned an error ABC:07.00.02.01:DATA:GL_DetailPosting:000000:000
The error was Error #15: Operating system command failed in GL_TableConversion0500.pvc at line 348.
View the Activity Log to locate a specific conversion log.
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From the log:
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11:35:26  Converting G/L
11:35:26  07.00.02.01:DATA:GL_DetailPosting:000000:000 started
11:35:26  Starting GL Detail Posting Check Number expansion for company ABC  (GL_TableConversion0500.pvc:00314)
11:35:26  The following conversion returned an error ABC:07.00.02.01:DATA:GL_DetailPosting:000000:000\The error was Error #15: Operating system command failed in GL_TableConversion0500.pvc at line 348.

11:35:26  Errors were written to the conversion log while converting ABC Company Data to version 07.20.00.00.
Closed log