2007 Mazda3 OEM Stereo replacement
 

I want to get the 6 disk in stereo OEM stereo to replace my OEM stereo. Would this be a simple swap?

Yup, less than five minutes and two bolts and you'll be done. Bear in mind if you have a '04-'05 model you need a '04-'05 HU. If you have a '06-'09 model any of those years will work (though with a '06-'07 HU you'll be able to unlock the Driver Information System.... read: mileage computer).