These files can be converted by dragging them all simultaneously onto the FM15 icon. Once converted, you'll want to test them to ensure that everything works properly.
From FM10 to FM15 you will just see a lot of benefits as a developer and as a user. You should consider migrating your files into one file, but that is not a must. Good luck with the conversion.
Here is a good page to read about things that happens to your database when going from FM10 to FM15
V10 to V15 is easy as mentioned by others
Take care of fonts... FileMaker changed the pdf engine (among others) and we've had some issues with some fonts (ie Univers by LinoType)
If You don't already have V15: Download the trial version, do a test-conversion (test means that You are using a copy, users will continue to work woth v10) and check the outputs, pdfs
V15.02 brings some changes - but I always recommend checking outputs.. (and the daily processes, of course)