Question regarding the actual mechanism of implementing proposed WCA changes

I'm trying to avoid the doom and gloom of the other posts, so I just wanted to check something I genuinely haven't been able to get information on and wanted to know - if Labour does go ahead with WCA changes (either the Tory's or others), how easy is that for them?

Like, can the DWP just change their assessment criteria on a whim so Kendall can just set whatever criteria she likes whenever she likes (I really hope this isn't it, but that's what some of the news articles I've been seeing seem to imply), or do they need to actually change the law? If its the latter, is this a case where the bill has already been passed by the Tories (and labour just needs to not cancel it before it comes into effect), or a bill is already in talks and we're seeing if it moves on to the next step, or would Labour need to present their own bill? Again, various news sites seem to imply various of the above without clarifying anything.

I've found a lot about the proposed changes but I've not seen a lot on what changing the WCA assessment criteria would actually entail, and I lack the legal knowledge to understand the more technical pages I've looked up. Anyone be able to give me an Explain Like I'm 5?