In fact, the EX4600 does use ELS, I'm not sure why it's missing from the feature explorer--it shares the same chipset as QFX.
test@test> show chassis hardware
Hardware inventory:
Item Version Part number Serial number Description
Chassis xxxxx EX4600-40F
{master:0}[edit]
test@test# set vlans vlan10 l3-interface vlan.10
error: l3-interface: 'vlan.10': Only IRB interface is supported, e.g. irb.10
Conversion is fairly straightforward unless you have an extensive storm control or spanning tree configuration. There are additional differences in where interface attributes are located as well as qos behavior between the two platforms. All in all it's not a huge ordeal to migrate. I'm sure folks here can help if you post before and after configs. While I haven't run 15.1x on the EX4600/QFX, my experience with it on EX4200/2200 suggests you stay as far as possible away from it and stick with the recommended 14.1X53 release for that platform.