Let’s just talk straight WP for a moment,
When you install WP it creates one user. The user has ID 1 and the name of Admin. ALL WP sites do this which is why you are advised to create a second user immediately and remove this pre-created one. In other words just use the ‘Admin’ user for initial logon. I don’t know if you have done this.
If you have not done this then in both your source and target the admin user will be assigned to ID 1. If the admin login names are the same on both systems you can not copy that user into ID 1 because (a) that ID is already in use and (b) login names must be unique.
So a lot depends on the IDs being currently used. You need to be careful as it is actually possible to lock yourself out of the site if damage is done to that admin data. So it is important to know what is what – to know what IDs are in use = particularly by the admin user – and then plan the data move carefully. It can be done easily – you just need to plan it out first and know what to do and what not to do.
