This post made to warn people who want to upgrade their EX3300 switches from 12.x software to 15.x.
It's obvious that you should upgrade your switches software and maintain it current version according to Juniper recommendations. Althoug Juniper recommends for users of 12.x maintain latest build in 12.x tree, I was interested to upgrade on 15.x and that cost me many hours of troubleshooting.
The new version is is aimed at enhancing security. And it means that some of configuration options is now applied more strictly.
I was using the neat feature called interface-range to bulk configure two stacks of EX3300 switches. Main point was in default rules propagated by interface-range (default access VLAN, voice-vlan) and override that came from each particular interface config, so that "default" VLAN was overriden by interface VLAN. As was stated by JTAC, in 15.x they removed configuration override so it's purely impossible to do that.
I've tried to upgrade my stack by different ways: NSSU, upgrade with service interruption, multiple different 15.x versions, etc... And every time my installation process was failed just because of configuration check false result. JTAC recommended to fine-tune my existing configuration on another stack, then apply it on my production and then upgrade to 15.x.
In a meantime I was thinking about how I can fix this. Separate configuration for each interface is not an option, so I see only two solutions: group configuration for each VLAN and interface-range for each VLAN. And I vote for second one.