According to "Upgrading to Unity 5.4" there are changes with how you sample unity_SpecCube1.

"Reflection probes share a sampler between the two textures. If you are sampling them manually in your shader, and get an “undeclared identifier samplerunity_SpecCube1” error, you’ll need to change code from UNITY_PASS_TEXCUBE(unity_SpecCube1) to UNITY_PASS_TEXCUBE_SAMPLER(unity_SpecCube1,unity_SpecCube0)."
It doesn't appear that we always use UNITY_PASS_TEXCUBE but usually use the other helper macros.

In the case of UNITY_SAMPLE_TEXCUBE we can just switch from UNITY_SAMPLE_TEXCUBE(unity_SpecCube1, coord) to UNITY_SAMPLE_TEXCUBE_SAMPLER(unity_SpecCube1, unity_SpecCube0, coord)

However there is no equivalent helper for lod samples. This exists UNITY_SAMPLE_TEXCUBE_LOD(unity_SpecCube1, coord, lod) but this doesn't UNITY_SAMPLE_TEXCUBE_LOD_SAMPLER(unity_SpecCube1, unity_SpecCube0, coord, lod)

Is UNITY_SAMPLE_TEXCUBE_LOD_SAMPLER a helper macro that should exist but doesn't? If it should exist there are various other texture types that would also need UNITY_SAMPLE_*_LOD_SAMPLER equivalents.