Transitioning dog to PPP-Advice

Hi there, I recently switched my dog to PPP Small Breed Chicken & Rice Shredded - Adult. She had been eating Health Extension Chicken & Rice Formula for 3 years until I discovered this sub and realized they were not WSAVA approved. She's almost 9 years old, 10 lb chihuahua mix, and has had a solid stomach, never had gastro issues. I switched her food gradually over 14 days- taking my time because she's so little- and stools have been normal during the transition period. Sunday night she went to just eating PPP, and today Tuesday she had a very very mucus-y poo. I only had a small amount of her old food left, so I mixed some of that back in with her bowl, but should I start back over with the transition process? Or see how the next few days go? Any advice appreciated, and of course if things escalate or I am at all worried about her well-being I will bring her to the vet. Thanks!