As an author of many heavily used tools on the FEX, I'll suggest that it is important to ask the author for their approval. (Note that this is only my own opinion, as an author.) Show what you did. Even then, issues can arise. Why do I say this?
You cannot simply update their code, on their submission, as only the author has permission to do so. So, the two options are:
1. To send your modified code to the original author, offering the improved version to them to repost, with thanks to you of course.
2. To post a modified version, under your own name, referencing your version. Explain CLEARLY what is different of course.
If they post your code as an improved version of their own code, then the author must now maintain it, and be able to treat it as if it is theirs. The problem is, most of the time that I see modifications to my code done by others, those mods are what I'd call hacks. I.e., poorly implemented versions that (probably) do what the modder wants, but are not up to the standards of code that I would write on my own. But if I post code with a mod in it, I will still need to maintain that code, as if it was my own code! So, before I'd post modded code, I'd need almost always to implement the fundamental idea of the mods done to the code so they were up to my own coding standards.
If you post a modified version, under your own name, but implying that your version is better, then it should indeed be better code. And you need to maintain it. Otherwise, you are doing a major disservice to the original author. I'd had people send me e-mail, asking for help on code of mine that they had modified, or been modified by someone else. The rule in free software MUST be that if you modify it, you bought it. Any problems at all in that code are now yours to own.