Update: Success. Previous "compact" tables are now showing as "dynamic".
I entered the following as an SQL query through PHPMyAdmin:
SELECT table_name, table_schema, ROW_FORMAT
FROM information_schema.tables
WHERE engine = 'InnoDB' AND TABLE_SCHEMA = '<YOUR DATABASE NAME>' AND ROW_FORMAT = 'COMPACT'
... then opened a new window to the same DB SQL query page, and copied the following query (adjusting the <compact_table_name> as needed :
ALTER TABLE <compact_table_name> ROW_FORMAT=DYNAMIC;
After doing this one at a time for the half-dozen or so entries I had to work on, I re-ran the original query (to identify compact row_formats) and came up with nothing.
Edit: And the Administrator Notification pointing to "compact" row_format issues, is gone.