Flashing it with any firmware is fine as long as you have the same CID. the important thing is the CID....
Flash it with the most up-to-date firmware. Use the search option, there's a lot of ppl that debranded their w580 already. They have given specific FS and Firmware files as well as the cuspack.
lastly, go READ TUTORIALS. Most of the info you need is already on the site. just use a bit of time, do some research. If not, take it to a shop and let them do it for you. IF YOU DON'T KNOW WHAT YOU'RE DOING, DON'T DO IT. (or say bye bye to your phone)