Does anyone know what exactly would happen if I do that update to the phone? Are there any noticeable changes (for example new content)? Sony Ericsson's site don't tell much about it, there is: "to keep the software in your phone up-to-date and ensure optimum performance from your phone".

as told, it would update the software of the phone which would give it a better and faster perfomance. its like refromatting your PC although you will lose some of the applications you installed on our PC. its better you update it so that some of the new apps that will come out will be compatible to your phone. i updated mine a little while back and the bugs and glitches associated with it was fixed. hope this helps.