and yes melidomi, i have big boobs and i rather to shape it then to swim inside, so maybe the better is to knit another bodice with another pattern which has already shaping for the big boobs and such then to take risk for the alterations?yeah, especially if you're planning to use a fuzzy yarn - that stuff is impossible to rip out.
i think the key to having a knit gown be pretty is to use a really fine yarn, so that the fabric is thin. Commercial wedding gowns magically make you look skinny and beautiful, and if your knit fabric is at all bulky it will do the opposite. The knit fabric doesn't have the same support as woven, so it can't hold you in place the same way. So you'd probably want to have some good undergarments - like a bustier/corset type thing - to get the 'bridal silhouette' - also, that gown looks like it's loosely enough knit that there's probably some kind of slip going on underneath it just to make it not see through.
Definitely keep us updated if you decide to do this! I think you'll find lots of people here with suggestions if you get stuck anywhere!