I don't like seaming so usually knit top down or bottom up yoked sweaters, or sometimes bottom up and do a 3 needle bind off for the shoulders.
It depends where you make your mistake and what it is. With a top down raglan or yoke sweater you can try on as you go to get the right size. You can do this with bottom up too, but it's a little harder to hold it up. I think the top down go faster.
The LYSO was right, you can do it in one piece to the underarm, that's a pretty basic conversion. It may seem like you're casting on a lot of sts at once, but it's worth it. I wouldn't sweat the possibility of making mistakes - you don't always have to rip out the entire thing to fix them.