Although the Support web does not really show this, for legacy EX products (EX4200/3300/2200/45xx) the [useful] code streams were basically 12.3 and then 15.1. Neither 13.x or 14.x (especially any X release) were targeted for these products.
So in reality 12.3 to 15.1 is the equvalent of "one" jump. So no problem going direct.
NOW, I am not sure you exact reason for moving to 15.1, but if it is just to get on JTAC recommended release, and you have no other pressing reason, and you could afford an upgrade window in the future, my (personnal) recommendation would be just stay on 12.3, which I assume your EX4550s are running fine with - yes?
12.3 has just recently had extended EOSupport until [at least] June 30, 2018. There is actually current work going on for a 12.3R13 [consolidation?] release. I am still hearing stories (true??) regarding general 15.1 instability with legacy EX products.
I might suggest for yourself (and others) that before you go ahead an upgrade any legacy EX product you have a discussion with your Juniper Partner technical people or your local Juniper SE. Might save you some headaches down the road.
Just FYI.