When did Sam Uley first become a wolf and imprinted on Emily Young?

I know the basic details: Sam Uley was the first among the Quileute youth to become a wolf and at some point, he imprints on Emily Young while he was still dating her cousin Leah Clearwater. What I want to know is WHEN does this happen? Was Sam Uley already a shapeshifter by the time he found Bella in the forest after Edward leaves her or does his first transformation and imprinting occur during the four months that Bella spends moping? Thanks!