I actually made this exact genre change with one of my own works...and I think the toughest part is realizing that it's not as simple as it may seem. ^^; At least, in my experience...I soon found out that turning the MC from a human superhero to an amnesiac zombie and tweaking the plot progression wouldn't be enough. Pretty much EVERYTHING had to be rewritten in order to fit the new genre.
For instance, the superhero trope of 'the charming rogues gallery' no longer worked now that the genre was more grounded. Neither did the equally tropey expectation that the other hero characters would get together and form a 'Justice League'.
I couldn't just sort the characters into goodguys and badguys and have the audience accept that. So I had to find actual reasons for all the members of my giant 24-member ensemble cast to exist in the story, and if not, give 'em the axe. This alone took a long time and several rewrites...it's hard to let go of side characters whose sole purpose is to be as adorable as possible in as little time as possible. ^^;
^And that alone morphed the story into something completely different. When I killed off characters, I had to write in moments that reckoned with that. I had to adjust character groupings, and mold their personality dynamics to fit the new groups that they were in. I had to pick just a few special badguys for the hero to face, and write in buildup to their encounters; character moments that lent context to their villainy, and reorganize the plot around them. I had to give the MC actual relationships with characters that were little more than flat henchmen in previous iterations, and raise the stakes.
The story isn't unrecognizable from how it was when it started, and a lot of the main themes are the same, but that's mostly due to the hours to weeks I've spent trying to figure out a way to keep something that I wanted to keep and still have it make sense in completely different circumstances.
And the process still ain't over, years later. ^^; At the moment I'm trying to figure out whether a former deuteragonist needs to be given the axe (which would be a HUGE change, but would ultimately streamline the plot) or if there's a way I can make them active in this new story, despite the actual MC having more of the focus. Like I said, it's hard. ^^;