OMG you've called upon me as if I'm an expert.
I wouldn't expect there to be any hem allowance to let down, so I'm not surprised by them saying that, but I'm not sure about the dress lengthening by the seams being let out? I don't understand how that would work.
How much would you need/hope to gain in the length?
Good news...The seams will almost certainly get you a little extra room, if as you say it's a bit tight. There's a possibility of the old stitch holes showing in the fabric, depending on the fabric and how sharp or not the needle was that it's been stitched with. Even with gentle pressing it can sometimes leave faint stitch marks.
Is the length really that bad/too short?
You may have to consider lower heel shoes if the length won't alter as you'd prefer it. Disappointing, maybe, but not the end of the world...
It's a beautiful and elegant shaped dress. It's lovely.
Oh, I suppose I should mention that I've been altering clothes for the last 38 years but not specifically bridalwear, so I'm not talking complete rubbish.
Sandra.
xxx
While I've been typing this there have been several other replies some of them saying bits of what I've said.
Also, I'm with fajita on her comment.