You certainly have to pay the full price for a FMPA15 release when you skipped several versions.
FM12 and higher uses a new file protocol (.fmp12) but current FM Versions will convert your old .fp7 files automatically. No big deal.
btw, FMI stated they would go with a annually update cycle. According to past release dates, FM16 can be expected to be released in May/June 2017