In this case, I'd just do a cold turkey switch
You may have some tummy upset (watch for that) but if she's picking through to the Pedigree - best to remove it from the equation altogether.
I'd also go with the tough love approach. Put her food down (all Acana) and leave it for 15 minutes. If she hasn't touched it, pick it up and she doesn't get it until next feeding. No treats, etc either in between. She'll get the idea.
If she's still really holding out - it could be she's just not a fan of the food. You may want to try a different brand/flavour at that point (but Acana is a very good food!)
Good luck!