It only means that the extended functionality you added will not stop to work but the look and feel will be the new 2013. however things that will be there and may be really irritating to some people:
The left menu will be gone.
the new top menu will be all over
few inevitable things like:
The section tiles will lose the arrow. it is still collapsible but you will have people asking what happen to the collapsing titles because without the arrow it looks very different
the form field labels from 2011 are 150px large and collapsible, when you migrate to 2013, you may notice some of your labels will be cropped. you will need to increment it to display the whole thing
I Hope I could help. If I have answered please mark as 'Answer'. If was just helpful, please vote. Thanks and happy coding! Bruno Lucas, http://dynamicday.wordpress.com/