I have to say that I don't know what the team will do. There are some who have strong (sometimes negative) opinions about each of the three centers, often depending on the "what have you done in a game lately" method of giving voice to that.
The issue from where I sit is that if they were to move Mittelstadt, they are back to not having enough NHL centers. They have four right now. Many teams have multiple centers playing the wing - think of Edmonton where RNH is on McDavid's wing instead of running his own line - rather than having to scramble to find one if any of the others go down. Mittelstadt is well liked per everything we've seen and what we've heard as sort of the guy the younger players hang with near the lake. He puts in the work, he's diligent and while not a banger plays an active puck pursuit game and is dogged at trying to recover or win pucks. It's hard to just pull that out of the lineup for even more futures. I like Krebs, I am not comfortable right now pushing him into the role Mitts fills. As for Cozens, it's hard to deal a guy who just put up 30 as a 21-year old who has the tools and projectability he does, even if they were to move him to the wing. Thompson is coming off a 90-point season and while his production is down, he's had some injuries he's tried to work through including coming back to play after McAvoy kicked him (and then later in that game breaking his wrist which he then returned from a few weeks early).
The Sabres biggest need is for a partner for Dahlin or Power (and I'm not partial if it is a LD or RD, though of course RD would be preferred) who can handle defensive situations, PKing, the takeout portion of a high-level takeout-breakout pair. And after that, experience for their wingers and younger players who get caught being young often.
Goaltending has solidified with Luukkonen. He's not Vasi or someone like that, but he's giving them quality starts in a way they haven't had from anyone since Ullmark left (and perhaps even before that since Ullmark was so often either injured or in his own way not ready). Levi continues to look very promising and needs practice and game reps IMO to round out the parts of his game that need work.
So the short version would be continuing to hang onto them all. I know the Avs are looking for a 2C and honestly, a pursuit line with Mitts and Lekhonen would be a serious PITA for a lot of teams to deal with. However, I also don't look at what I would think the Avs would be willing to move and think there is much there that moves my needle - Buffalo is at the point they need to find their own Lekhonen types, not chase more prospects or draft picks. Just my two cents.