updating wordpress when running 2010

Hi Everyone

I started out on wordpress.com but will soon be moving to ww.wp.xz.cn

My question is related to wordpress’s own default theme 2010.
It says under updating wordpress in (docs) that if you update 2010 and 2011 wordpress themes the site/blog can be affected or you lose the changes you made to the site!!!

I will only be changing the site in terms of using widgets and all the other simple changes like background colour and images I wont be changing the code in any way or doing advanced stuff just doing what I was doing in wordpress.com.

I am a real novice and still learning but really do love wordpress !!

Do I still have to worry in case my site keeps altering when I update to the latest version of wordpress all the time???

Or will it keep switching back to the default setting????

Any advice or help is much appreciated

Thanks so much everyone.

Bob