Is this a normal reaction to food change?

My newly adopted dog is a ~8 yr old pitbull and has food allergies. We moved him from Purina One Vibrant Maturity to Natural Balance Limited ingredient salmon and sweet potato. We transitioned him slowly and he's been on the new food fully for a few days and his skin bumps have gotten worse. Is it normal for food-related allergies to get worse before they get better when switching food? How long is too long to still be having a reaction when on new food? Also, his gas has gotten better and it seems like he wants to drink water more regularly now whereas before he wasn't wanting to drink as much. Thanks for any help!