Yes many things will change, mainly how your user interface looks.   Apps and files should remain.  I won't try to downplay your worries about everything you mentioned above. It is a risk, especially for some users who are updating to 5.0.  But, in the end, the update will install itself in time.  The best course is to backup your files locally to a PC if you do not want to use a cloud service.  You can defer the install only so long before that option is gone and you must choose when to install, as in what hour, not what day.  Do not root.  You will void your warranty.  And, you have stated you have no interest in hacking and such.  Stay away from that.