Will you be hosting the file with Server? If so, you might want to review some of the posts on Server over in Report an Issue...
FMP 11 database files convert to FMP 13 pretty cleanly. Odds are good that you will not need to make any immediate design changes to your files to get them to work correctly after the conversion is made.
But the user interface for designing layouts is quite different. It takes a bit to get used to as many little details (such as what happens when you drag a selection box around/across objects on the layout) aren't the same in the new version and you also get quite a few new features not available to you in FileMaker 11, such as themes, popovers, sliders... etc so it will take some time to get used to.