Well, it's probably best to still do a gradual switch - unless you know your dog is pretty resistant to food changes (and many are!). The chances of having issues changing between these two formulas are less than might otherwise be the case - not just because they're the same brand, but because the lamb version contains nothing that the original version doesn't (ie. it just leaves out the chicken/turkey/herring). But that doesn't make it impossible that a fast change can cause some tummy upsets. So if you dog is the sensitive type, or you just want to play safe, then a gradual change-over is still the best way to go.