Learning: Combining successive ALTER TABLE queries into a single combined query reduces execution time dramatically (~1.95x speedup) by minimizing I/O, lock acquisitions, and index rebuilding time for database migrations on large tables. The yourls_upgrade_to_506() upgrade path has been optimized accordingly without losing or altering any table schema state.
Action: Refactored includes/functions-upgrade.php to map individual URL table modification queries into a single concatenated ALTER TABLE statement, retaining the sequential constraint modifications (CHANGE keyword, CHANGE url, CHANGE title, CONVERT TO).