Actually in this case you can upgrade directly from any 12.3 release to any 15.1 release. I might suggest you remain on 12.3 for a while longer, as 15.1 has been shown to not be 100% stable with older legacy EX products, EX2200/EX3300/EX4200.
Support for 12.3 has been extended across of these products mentioned until [at least] Jan 2019.
Just FYI.