What is the best way to merge two disjoint tables?

Hi Gurus,

I have two disjoint tables, for examples, of user lists.

Some users appear both tables. They may or may not have the same u_id.
Some users only appear one table.

I would like to merge tableB into tableA with the condition that each
user has a unique id.
Also I would like to keep u_id in the tableB. This requires to modify
u_id of tableA if there are matching users but different u_id.
Also those users only in tableA should change their u_id, if needed, to
avoid any conflict with u_id from tableB.

Can someone show me an example SQL statement?

tableA:
u_user | u_id
----------+------
user1 | 46
user2 | 30
user4 | 22
user5 | 48
user7 | 6
user9 | 1
user11 | 45
user12 | 33

tableB:
u_user | u_id
----------+------
user1 | 46
user2 | 30
user3 | 22
user6 | 48
user8 | 6
user10 | 1

Thanks,

- Chansup