Ok, if you look at the box of a Lightcore figure that's original figure was in SSA, it says that it won't work on SSA, only SG. However, according to the offificial website (scrolldown a bit):
So I guess Reposed LightCores will work in SSA, just without the LightCore bonus ability (also, as is easily guessed, Sereis 2 will not have Wow-Pows in SSA).
Having learned this, my mind has been settled and I bought LightCore Prism Break.