Ok, Luke @Luke, I’ve got you (and anyone out there who wants or needs to lower the pitch of a reed) covered!
Use this 8 minute video to see how to do this (and save me from typing a very long post)
Be sure to watch the entire video before doing anything on your own. I suggest using a good tuning app to check your progress, but doing as he did in the video is also good and using both is the best. As for a file: I believe the Lee Oskar tool kit includes one. Otherwise buy or use a cheap slender file (as in the video).
I also suggest that to lower the pitch you remove material over the first 1/3 to 1/2 of the length of the reed (at the rivet end ). If you remove too much and need to sharpen the note, remove from the far end (again 1/3 to 1/2 of the length). In other words: when lowering pitch, do not keep filing away at just one spot because the reed will become too weak and will fail sooner – use the entire 1/3 to 1/2 of the reed length.
Also: only file lengthwise (in the long direction of the reed), not crosswise (which also would shorten the life of the reed by weakening it).
If you remove too much material and need to sharpen the reed, do not file the very tip of the reed (it can be done but is tricky and requires more skill and caution). It is better to stop and check your progress regularly – before you remove too much material!!
Needless to say, all of this also applies to blow reeds – but then you must remove the blow reed plate from the comb.
I wanted to re-post this awesome advice from @Slim (thank you!!!) regarding Tuning harmonicas in it’s own post so that it’s easier for people to find it, and we can keep the conversation going (and be on-topic. )
Thanks so much for posting this @Slim, will keep you posted as progress is made…