Appears as though you've got a well constructed deck there. I'd advise if you'd want to replace the center that you could easily switch to an exterior rated LVL laminated beam. It would be difficult to remove the 4x4's currently and use a 6x6. If you must I'd use a recipro saw and cut the fasteners if possible or at least make that part of the plan if you can't get the current screws out.
http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf
^^ is a good guide for deck construction.
Sorry that code isn't truly uniform in the US... at least whomever you deal with as far as permitting should be helpful and pleasant about it.