alt Hacker NewsCourse credit is not the same as professional certifications. You can apply past expertise/work to as many different certifications as you can manage.
Submitting previously graded work for a new course wastes your own time. It also wastes the professor's time, because they assigned that writing for you to develop a specific skill, and you're trying to not practice whatever they're trying to teach you.
If you have a perfectly matched essay to the assignment, you should talk to the professor about how the assignment can be adapted.
I see where you're coming from, and perhaps because I'm absolutely not cut out to be in academia, that's where my bias comes from -- I'm inclined always to view education as a means to an end, and that's certainly far from accurate.
The interesting thing is that in coursework context, plagiarism of any type is merely a massive self-own, since you are (probably) paying money and then purposely cheating yourself of the learning that you paid for. To me, I feel like it needs a different name than plagiarism in the academia or professional world, where you're taking ideas from others and slapping your own name on them for clout or for money. In this case, a real victim exists and you also stand to gain from it if not caught.