Any seamstresses out there? Wondering if altering these dresses is a bad idea.

So I searched everywhere for a high-low bridesmaids dress option with a high-low skirt (I know, it's so blurry but it's all I have.) My wedding will be black tie optional and the entire aesthetic is glam, but my wedding is also in May. I wanted my bridesmaids to be very dressed up but not too hot.

I found gorgeous black Vera Wang dresses at David's Bridal instead, and they are floor length but the bottom have alternating sheer and solid panels. I was thinking about telling the girls they could alter this underskirt and cut it to above their knee. While I was trying it on in the store, I pinned up the under layer to above my knee and walked around, and with the sheer panels at the bottom it felt much cooler! David's Bridal doesn't offer that kind of alteration, but they could take it to a local tailor for that.

Does anyone with sewing/dress design experience know of a reason I shouldn't allow the girls to shorten this under layer? Obviously it helps conceal the lines from undergarments, but if it's left to the knee it would still do that....thoughts?