Can i put in a plunge on my wedding dress?

I would double check with alterations to be sure the structure of the dress can handle that deep of a cut. While it might be similar, how it's made will determine that (you wouldn't want to take away a lot of fabric and it sit kind of frumpy because it won't hold shape now, the gap won't stay closed and just kind of flaps about showing everything off, etc). If not there may be a way for them to add support so that it can, but as PP said, I'd definitely check with the local seamstress.

I agree it is going to depend on the current structure of the dress, and what additional structure might be needed if the currently neckline is altered. Only a very experienced alterations person can give you an idea of what is realistic, and the cost. Almost anything can be accomplished, but the cost (based on work & materials) might be prohibitive. FWIW I think you look beautiful in the dress as it is currently.

Yes! This is possible! I literally went through the same thing and asked that in my post last week! I bought a dress and loved the lace and detail but wanted a plunge instead of a sweetheart neckline. I called the seamstress and sent her pictures and she said yes, it was possible to do!