Can you alter a chiffon dress with a straight neckline to a sweetheart neckline?

It probably can? But having a photo would be helpful You could also just bring it to a seamstress and see what they say - then you could get a quote as well.

With the rigth seamstress almost anything can be. chiffon is a tricky material so talk to someone who knows their stuff. get a price quote from DB as well and start comparing!

If I was the one to do it I would look mostly at the material. the more flimsy and thin the material the more I would say no. but either way I wouldn't call it impossible.

I would just speak to a seamstress and see what they say about it.

Mine was a DB chiffon dress, straight neckline. They pinched it into a sweetheart neckline for me.

Mine is chiffon over satin and they are fixing the neck line into a more pronounced sweetheart for me.

I'm on my laptop now so I can add photos. Here is a before on the model and an after at the wedding.

Mine couldn't be (but could to have a slight dip), but, some can. Talk to your seamstress.