Would this be a good idea to handle the problem with my dress?

I need to get a dress for a church Christmas banquet, but I'm worried a short dress will be too informal (it's a "night at the Oscars" theme....the youth pastor's wife is wearing her WEDDING dress). However, I don't have much money to spend on a dress to wear once.

I'll need a nice dress to wear to a few upcoming occasions, but nothing as formal as the dress I'll need for this dinner.

I'm thinking about sewing the dress and then after the event shortening it a bit to make it more semi-formal. What do you think? Would that be a good way to handle this problem?