Is it a scientific misconduct to change a variable or a function name when citing a math paper? Is it a scientific misconduct to change a variable or a function name when citing a math paper? The standard name of a function is S, but the author called it E. I want to cite his work, while I use the standard S.
It's certainly not a misconduct. Different authors use different notations. I've even seen different notations being used in papers from the same author. If the new name helps the reader understand what the function is supposed to do, or if it's a newer standard notation, then you are perfectly allowed to change it. In fact, I would encourage you to do so.
I would simply cite the work, write the formula and add a footnote on the lines of

¹ For consistency with the rest of the manuscript we renamed the function E in [1] as S.

Or

¹ Note that in [1] the function S is denoted as E.