I forgot one thing: As a kind of step #8 at the end:
Since a switch from vb
to phpbb3 will result in different usergroups within phpbb3, they do not match anymore to the old settings in the ACP of RP, PP, CL. So you should make sure to refresh the usergroup within the RP, PP, CL ACP and check the usergroup rights there and save it.
If you forget this step (I made this mistake), you might be able to login, but you most likely can not upload products/ photos/ads, since the usergroup for this username has no settings/wrong settings within the ACP yet