You could cut adult food into the puppy food and watch the dog's growth.
I also thought Diamond had reached her full size (her Dam is normal GSD size, but her Sire is HUGE) and was ready to stop feeding her puppy food. BUT, she's having a growth spurt now, and has gotten bigger still.
I think I would wait a little while before completely cutting out the puppy food. Giving 'some' won't hurt, but stopping prematurely can make her growth uncertain. Your pup is only 8 months old (Diamond is nearly 10 months and growing), so I'd give it another month or two, just to be certain.
You can also talk to your Vet again. Get more advice.
"Always be a first-rate version of yourself, instead of a second-rate version of somebody else." - Judy Garland