If you're going to change her food, it really would be better if you were to switch to a higher quality one. Pro Plan is not the worst food on the market, but it's not great either. It simply doesn't contain enough meat for a canine, and it's in her long term health that you're going to see those effects.
It's not as if it costs significantly more to feed quality either. Sure, the bag costs more, but you feed so much less of a premium food that it works out to be fairly comparable - in some cases cheaper.
Still - to answer your specific question: yes, it would be wise to approach a change in flavours exactly the same way you would a change to a different brand. It's an entirely different food, and since your dog has been eating the same thing every single day for four years straight, she's probably more likely than most to be sensitive to dietary change. A gradual change is therefore wise.