My girl switched to size 1 at 2.5 weeks. Basically if it isn't too big on em, I try it. I'd rather have too big than too small
For brand like Pampers and the Walmart brand, there is a indicator on the waistband for when to upsize the pampers. But just a good rule of thumb, if tabs are nearing the end of the waistband, upsizing may be helpful.
It’s the blowouts / leak and where the tabs are placed. If it’s on the edge vs the middle yes move up. Go by weight scale on the package also to help. Some baby are bigger so may move up faster.