I tried to open 2013 file on 2012 software and it didnt work. I dont think solidworks has an option to down save the file like the Graphic design Software have.
it was mentioned in the 2015 beta forums - SW apparently had to change the file format too much to keep that feature working. at least between 2014 and 2015, might work again for 2015 and 2016.
You should be able to open a 2015 in 2014 SP5