Hi Gem
I wouldn't be in too much of a hurry to change from the existing diet unless it was one which I really disapproved of (in which case I'd have been unlikely to be getting a pup raised that way

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The change in store for a new puppy, i.e. lack of mum & siblings company, travelling from breeders, arriving at strange and (to baby puppy) scary surroundings with new people etc can all affect a tiny tum. Not least, if you stay outwith the breeder's local area, your water may well upset the pup's tummy initially too

Add to that everything else the puppy has to experience which will be so different from it's place of birth and there's usually a lot of stress in store so I believe we have to minimise the stresses where and when we can (diet being one of the few we can control

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I'd personally wait a minimum of 2 but pref 4 weeks after bring your new pup home before implementing major diet changes and then phase same in over a further 10-14 days

HTH, Teri