What Greg says after he turns away from Amethyst-as-Rose: "I know you're doing it. I wanna be friends again. I really do. But I can't let you do this to me again."
Pretty much confirms she's done it in the past.
Well… that doesn't confirm the shape shifting, just the 'getting close to her and isolating himself from everyone else'. the "I know you're doing it" is is because she implied she was about to shift before he turned around
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It's sort of like .. .. "I CANT let you do this to me" is something you tell yourself when you're struggling with a weakness, like say a toxic friend or lover who's fun but you makes you feel bad about yourself. I don't see Greg being weak about not wanting to see Amethyst shift into his wife. I can see him being weak about losing control of his life due to a toxic relationship with someone he likes to hang with. If it had happened before, I would have expected him to be angrier and say 'I WONT let you do this to me again'
Idk just noting there to imply the rose thing specifically happened before imo.