I got the following error message while trying to upgrade PhpBB 2 to PhpBB 3

Colliding usernames were found on your old board. In order to complete the conversion please delete or rename these users so that there is only one user on your old board for each clean username.

See the screenshot below

We have fixed the error by run the below SQL Query to PhpBB 2 Database

update phpbb_users set username='553' where user_id=553;
update phpbb_users set username='554' where user_id=554;
update phpbb_users set username='555' where user_id=555;
update phpbb_users set username='575' where user_id=575;
update phpbb_users set username='563' where user_id=563;
update phpbb_users set username='574' where user_id=574;

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