#1 not sure if you are aware but support for 12.3 code releases has been extended until [at least] June 30 2017. Based upon this, and I am unsure your exact reason for upgrading, but I might suggest you wait to upgrade. I do assume you are not having any issues with 12.3, as most customers are.
Now if you want to upgrade to 15.1 you can go direct to 15.1. Reason is that none of 13.X or 14.X really applied to legacy EX products so going to 15.1 direct from 12.3 is basically a one hop upgrade.
Hopefully this helps you.
Now if you want to upgrade to 15.1 you can go direct to 15.1. Reason is that none of 13.X or 14.X really applied to legacy EX products so going to 15.1 direct from 12.3 is basically a one hop upgrade.
Hopefully this helps you.