can i get a different visa during my stay in china while the tourist one is still in effect?

Hello, I am a Portuguese national, I'm planning on visiting some friends who live in Beijing in October, i have already stayed there for a year from 2012 to 2013 studying. Now i want to return for a couple of months, and perhaps find work. If i find an employer that can arrange for a work Z, would that overrule the Tourist one? Or would i have to wait until the Tourist one expires, then get the Z type? And regarding the tourist type, is it required that i have a return ticket booked prior to applying for it, and does it dates get settled according to my flight booking details, or can i worry about that once im in china?