FileMaker 11 clients will not be able to access a file hosted by FileMaker server 12. Filemaker server 12 will not be able to host files with the .fp7 extension as FileMaker 12 uses a new and different file format.
To convert an FMP 11 file (.fp7 extension), you launch FileMaker 12 and use open from the file menu to open the file. The conversion process leaves your original copies of the file unmodified, but produces a new copy of the file converted to .fmp12 format.
Before spending $$ on server 12, I recommend downloading the 30 day trial of FileMaker Pro and using it to convert copies of your files. You can then test these converted copies to see how they run on your computers. You may also want to access the Known Bug List here in the forum and navigate to the last few pages of this thread to review reported and publically acknowledged issues with Filemaker 12 to see if any will adversely impact your solutions.