WP_w3all plugin will switch to integration by email: this is why and what it mean

[axew3]

The cost will be one: integrating users between WordPress and phpBB with this plugin, will require to choose that users can manage on their profiles, the email change, only in phpBB or WordPress, and users can register only on phpBB OR WordPress. Not on both.

So, the plugin leave the concept you see of this online example, and users will be able to modify their email, in phpBB profile or WordPress profile.
This will be easy to switch to this the actual code. And actual installed integrations, it's quite sure, are using or phpBB login/registration/profile update or WordPress side login/registration/profile update, not both like this example that is probably the unique one.

To avoid user's email change on phpBB profiles is quite simple, it is just necessary to deactivate a module into ACP, and setup ACP as usually indicated on the install help. So substantially nothing relevant change.

On the other side, WordPress side, so in the case the integration let users modify their profiles into phpBB, to not leave users edit their profiles in WordPress, an option will be added, that if enabled, will automatically redirect users to their phpBB profle, in case of email change.
It is also an easy do. Can be applied even something more easy, in many ways. Examples will be provided.

What benefits?
emails are indexed into both WordPress and phpBB databases. The integration will result much more fast when profile update functions calls, and when any instruction that require to search for users data fire: into a little database this may do not make a big difference, but where thousand (maybe millions) of users exists, this make a big difference instead.
Further more, since it is assumed that unique accounts can be created and exists, there is no need to check for duplicated values into phpBB when user's profile updates occur: native WP functions, will throw the error, and no update will be done by the plugin, because it will be subsequent, and if successful: all the check functions have been so removed, the code execution will decrease db calls to the minimum when user's profile updates occur. Same goes for phpBB profile updates, done on phpBB side. phpBB with proper options setup on ACP, will avoid duplicate emails.
Easy as is.

Usernames will not matter anymore: an username TEST with email test@myemail.com in WP, and username ANOTHERTEST with same email test@myemail.com in phpBB, can login on both phpBB and WordPress. Substantially association of users will be by email, not by username giving some possibility to integrate in different ways and different scenario, included where there are old phpBB and old WordPress installations, with existent users that may are with pairs username/email not matching on cms (in this case, users can login by email, if same username do not exist in WordPress. The coming phpBB logged user, wll be recognized by email, and logged if an user with same email (but maybe with another username) exist in WordPress). So nice.

This the reason why, from 2.4.0 > will be MANDATORY to choose: users profile fields updates (email), and registration, in WordPress OR phpBB, not on both. These are the unique relevant aspects for old plugin users, all the rest just will be as ever, but faster.

Any function will be rewritten and all the 'not useful code', unified or removed.