I recommend calling FM for that information. I believe there is an expiration date on upgrading an older version to a newer version and the upgrade policy usually only goes back so many versions. Such as 12 is the oldest version you can use to upgrade to version 15.
FileMaker 11 is not supported or "upgradable" any more. Therefore I suggest that you buy FileMaker 15.
Your 11 (fp7) file will need to be converted to a FileMaker fmp12 file (the format of FileMaker 12, 13, 14 and 15). This is done very easy, just drag your fp7 file to the FileMaker Pro 15 symbol. If you need to convert a solution with more than one file, please place it in a folder and drag the folder to the FileMaker Pro icon to have them all converted with correct internal links.
I would update to FileMaker 15. You need to convert your file to new standard, but that want affect your database that much. You can read more about that here