I'm afraid I'm a little lost here.
You said the MC who dies is just the MC of a subplot, but the way you're describing it makes it sound like you have a complete storyform for this "subplot" already selected, which makes me wonder whether it actually is a "subplot," or is simply a separate story that's been tacked on to the main storyform.
If it is a complete storyform unto itself, then I would caution you against changing the MC and IC halfway through. Handling a major character death is already complicated enough without switching perspectives along the way.
You can, of course, write whatever you want. It's your story, after all. And if you just mean to create a fun subplot where interesting things happen, I'd say don't worry about it at all. But if you are trying to tell a story that makes an argument about how to resolve a particular inequity, doing what you've described will muddy the argument and weaken whatever point you are trying to make. The MC and IC throughlines are more than just characters. They are perspectives on an inequity. They need to remain consistent, or your audience will have a very hard time determining what problem they're looking at.
Is there any particular reason this story needs to unfold this way? Could the character you've initially identified as the MC actually be the IC the entire time, albeit one we're particularly close to?
If the subplot is actually a subplot, not a complete storyform unto itself, then what you described in your post would be more doable. But in that case, worrying about who changes/ who's a do-er and who's a be-er is likely irrelevant. You're just throwing in an extra few chapters for color, excitement, and pathos, which is relatively low-risk. Provided it doesn't weaken the main storyform, then do whatever you like!
Best of luck,