If she's going well in a snaffle she should have no problem going into a full bridle (am assuming you know what you're doing here). I've never had a problem with changing, except where the horse had contact problems already. Some horses can be a little overreactive to the curb (I'm being polite; it's only TB's that are that dramatic really), but I have found the vast majority of cobs go vey much better in a curb bit than in a snaffle. Good luck with the showing!
I popped my dressage cob in a double this week. He's a level headed sort (most of the time!) so I just got on with it and have got some decent work out of him. I even contemplated competing in it this weekend but chickened out purely as I don't feel capable yet - he'd be fine though!
Depending on what your set up is I have heard of people hacking in a double initially just so you're not asking too many questions. Think this is a great idea but my ground for hacking is awful at the moment.
I just got on with it. Chose a bradoon similar to his snaffle and a weymouth with not too long shanks and off we went! I was careful to keep the curb nice and light and my pony was fine.