BuyItForLife

Aight, since you needed to ask, chances are that you can't handle doing it, otherwise you'd be asking what methods to use, right?

But it is possible to DIY that job. It's even possible to do it without losing a lot of the hardened steel, and without majorly changing the profile. I wouldn't advise doing that job as your first effort, but it is totally within the range of skill a beginner can develop with a little practice before starting the actual job.

But, being real, just find a pro.

Caveats: there's very little in the way of proper certification for being a pro. Any rando with enough money to buy a belt grinder can claim to be a pro, and there's not usually any way to prove otherwise or hold them accountable for being full of shit. And, even an actual pro with a decade or more of experience might not know how to handle that steel in a way that keeps full functionality.

Tbh, while I could do it, I'd be shitting bricks the entire time because it isn't exactly something a skilled hobbyist does often. Like, I've done it maybe a dozen times overt the last thirty years, and it was only the last few that I felt fully proud of the results. It's all hand work if you want it done right, mostly. You can use powered tools for some parts of it, but you shouldn't because the faster the tools, the faster mistakes get out of hand and ruin things. Again, I've done this kind of thing, and I still wouldn't want to use power tools for anything but the very first stages. And I'd never, ever use a belt sander btw. Don't trust anyone trying to do so with a heat treated knife unless they have a proper cooling system for their gear. I sure as fuck don't lol

Anyway, mostly useless I guess, but point is that the knife isn't trashed. It did lose life span, no way around that, but it can be brought back to useful

Edit: checked the comments. What you're dealing with is a knife with layers.

There's a core of hardened steel, with a softer steel (though iron is sometimes used) on the sides. The hardened core may or may not extend all the way to the spine, but the typical Japanese knives I've seen don't. Hence the issue with just hogging off metal willy-nilly. You grind down too much, and it isn't impossible to not have any hardened steel left to work with at the kind of angle that a know if that style is really meant to have.

You can see, even in this pic, where the type of steel transitions. I doubt a good pic would show it, but sometimes, you can look at the front of the knife when it's a nakiri and see how much core there is. If it actually extends to or near the spine, there's a lot less issue with just grinding past the chips.