I'm playing with the idea of saying "I'm really uncomfortable talking about my weight. [Obvious subject change]" Because I am, and because it's really rude to comment on/ask about other people's health. And maybe a gentle push will help people behave better.
What the subject change is will vary with context. At work, I'll ask some question about a work project or redirect back to whatever we were talking about before.
With friends ... well, with close friends, I actually won't have this problem, because I'm being honest about what's happening in my life. But with friendly acquaintances, I'll ask them what TV shows or board games they're enjoying lately, something like that.
With family... that's going to be hard. I don't see most of them that often, so it'll be a bit of a shock when they see me, if I'm suddenly down 100 pounds (not outside the realm of possibility, with my in-laws--I see them once a year). I might get my spouse to run interference with that side of the family, to avoid awkward conversations. My brother knows I'm planning to do this, and so does my aunt. My mom... oof, that's going to be an unpleasant conversation, probably. I'll see her a little under 2 months after the surgery, so the loss (and my changed eating habits) will be noticeable. (I might not even be on solid foods yet? So. That's fun!)
Anyway, what I'm saying is, it's your choice what you tell people. I'm not willing to imply that I did it all with willpower alone, because that is cruel to other fat people. But I'm absolutely willing to deflect and refuse to answer.