I did an upgrade project with a lot of version 10 code up to 12. The most reliable path was as follows.
Move devices to 10.4 version
upgrade from 10.4 to 11.4
upgrade from 11.4 to the desired 12 version
As mentioned you will need TAC help to get code from 11.4 on back. Contact customer support even if you don't have a contract. There were a lot of EX devices that were sold with complementary hardware only support and you might get lucky.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home