I have 2 boards, both on version 3.0. And I want to update one of them to 3.1.x and second leave on 3.0 (its heavily modified). But when I tested install phpBB 3.1 and 3.0 in one database and merged by this tutorial https://deconf.com/two-phpbb-forums-with-same-users/
I checked, that phpBB 3.1 used different auth system.
So is any option how to solve it?