I think of patterns as suggestions. Like cooking, some people need to follow a recipe exactly, some people hardly follow them at all.
A hat is generally simple. But if it has a stitch pattern that is multiples of 24 or something big like that adding one group could be too much of a size change. Adjusting needle and/or yarn size might be better. You may also have to figure out a different decrease to hold the same shape. If you just added a few stitches you could probably get away with bunching up the left over stitches and not worry about how they change the decreases, some hats have no or few decreases that rely on yarn’s give to bunch up the top.
I don’t generally use patterns and use this for my hat decreases.
Sweaters are more complicated. Shorter or longer arms, bust or belly darts may not be a big deal. But you probably don’t need everything larger and how do you know what you need larger without trying it on first?
Longer from neck to armpit on a raglan without increasing the body diameter or vice versa may require a different increase/decrease rate. Yarn is very forgiving so small adjustments can be made without worry about what it does somewhere else. But if too much needs to change you could end up writing your own pattern with the basic idea of the one you’re going off of.
Sweater Design in Plain English by Maggie Righetti is good. It can teach you how to make up your own designs to fit imperfect bodies. If you can do that you might be able to spot aspects of other people’s designs that you’ll need to change before you make it to try on.
Since yarn gives often close is close enough.
If you get something that just won’t work after you’ve knit it you may be able to steek it. Sew it up so it does fit and cut off the extra. Scary, I have a cabled sweater with long armpits, I don’t want to rip it back that far, may as well just knit a whole new one, but I’m also afraid to steek it. One of these days.