What is the purpose of changing serum (FBS) concentration from 10% to 2.5% while transferring the HeLa cell culture in 96 well plates for luciferase assay?
It may be that you're basically synchronizing cells for homogeneous cell cycle start, given that in cell culture each cell is in a different process of the cell cycle, some in mitosis, some in S phase, etc. This could perhaps increase transfection efficiency and expression of the vector. It could also be that serum could be inhibitory for some transfection reagents.
Yes the serum can interfere with some transfection reagent but, there are now commercial product that are less sensible so it could also can be useless to do that.