Default newbie help please!!

I've never flashed any rom on any of my android devices. I just bought the new Xperia Z from Bell Canada and found out that it is still on 4.1
No updates from the career for more than 6 months.

So I'm thinking of doing this - please help me

use this Rom from XDA

Use the flashtool described in this thread and do an update of 4.2 with default settings of the flashtool.

Hopefully it works just like that..if not, i'll download the bell rom (4.1) from xda and do the same with the flashtool to get back to where i'm now

Is this a good idea? i'm not sure loading the 4.2 will do something to the baseband and kernel that will stop me from reflashing it back to 4.1 if things go wrong.

any advise would be appreciated.