I remember them stating in a webex back when 15 was first announced, that if you built a 12 sp4 you could go to 15 but not any version prior. Those would require a rebuild, though that could have changed since. On the other hand 12 is still fully supported, with SP 5 current and I believe there are plans for 2 or 3 more SP's for 12. We have applications that will continue to run on 12 because not all the packages from 12 are available on 15 for one reason or another.