Trumpet to a-line alterations

Could the dress be custom ordered with a different dress shape? It'd probably be cheaper to special order it than to change it in alterations. Otherwise, if you can't order it with a different shape, it depends on the dress as to whether it can be converted, but it's possible with some dresses! I had my dress altered from a mermaid to a slim A-line. I had purchased extra lace fabric from the designer, and my seamstress was able to change the shape using the extra lace. The seams were totally hidden due to the lace, and you couldn't even tell that it wasn't the original shape. However, dresses without lace or dresses made with certain fabrics may be more difficult to alter to an A-line. Definitely check with the dress shop and with your seamstress to find out if it's possible, and if it is, what all you would need (likely extra fabric).

You will need extra fabric. Like PP said, see if you can order it from the designer. You will not be able to do it without extra fabric, and ordering the fabric from the designer will make it look the best!

Have you actually tried the dress on yet? Sometimes dresses are far more, or less, flattering than we think they would be from the pictures!

That would be a really difficult alteration change. You would need to order extra fabric.