Strapless dress/sleeves

I have purchased the below dress and LOVE it, well everything but the detachable sleeves. My goal all along was a long sleeved dress, particularly off the shoulder. I was told the seamstress could do this no problem. Whelp, I went for my first alterations appointment this past Friday and although the seamstress said she could do it, and would use fabric from my dress…. she still seemed a bit unsure which is making me nervous now. I still want them detachable like the original. Has anyone done the off the shoulder
LONG sleeve with their gown? I’ve been looking for pics and I’m having no such luck.