Generally, you should do it gradually (roughly one feed a day, increasing each day). But if you really think it's the specific brand (you've tried him on another and he's fine with that), then I would do it a bit more quickly. We switched gradually from Cow and Gate to Hipp, but my daughter had an awful reaction to Hipp (gave her terrible reflux and projectile vomiting, which she only got after those feeds and it escalated as soon as she was 100% on Hipp). So I switched her back to Cow and Gate immediately rather than gradually and her issues disappeared. I didn't want to take any more time to do it in that case because she was being so sick.
Are you sure it's the specific brand and not just formula in general? Formula is rough on baby's bellies and does tend to cause more discomfort than breast milk (which could be made worse if you baby has a dairy intolerance or a reaction to something in the formula, there's weird stuff in there, including fish!). It could also be something entirely unrelated to what she's eating. Some babies just seem uncomfortable while their digestive systems get up and working.
If you do want to switch, you might just want to buy some of those little ready to feed cartons and try out a few different brands and see what works best before you commit. Aptamil is generally considered one of the better brands, but you might consider trying Hipp. Most everyone else says it's gentler on tummies than any other brand. We had a bad experience, but most babies seem to really get on with it. It might be worth a try?
