If you're still in the barber's chair, you may be able to simply ask for a touch up. Do so only if you're completely confident this barber is skilled. That said, if your haircutter is skilled you're probably not going to be asking for a touch up, so it may be best to cut your losses and find another barber.
When you go in for a touch up, let the barber or stylist determine the best options. Sure, you might end up with a much different style than you're accustomed to, but a skilled haircutter will know what's best.
Look on the bright side. Fixing a bad haircut may force you into stepping outside your comfort zone and trying something different for a change. Change isn't always a bad thing.