Default [Q] Change DPI without root !

Hi every one
first, I'm sorry for my bad english
We know that we cant change dpi of our devices without acess to root permission. I have an idea
I have an xperia z and if i root it my warranty will waste. i'm going to flash the stock fremware (FTF) with flashtools
And i can open FTF file and add or edit anything on that. (with Winrar, yaffey & ...)
if i open the build.prop and change the dpi line (ro.sf.lcd_density=xxx) and for example set it to 340, and then flash ftf with flashtools, will any problems arise?
Anyone have experience?
if is it unsuccessful i will update my phone to KK fremware that sony released. because if i flash my phone i will lose data on it
the default dpi of my z annoy me because i thing anything if very big and i feel that my phone is small !
thank you for all your Comments