I do the same. I still book the new one before cancelling the old booking, just in case it doesn't book properly. It's not worth taking the chance of cancelling first, just to get the app vouchers. But, if it's a SNU booking, you have to cancel it first to rebook it!

What I often do, is if the original booking in in my account, I will book a new one in my wife's account, or vice versa. That way, making the booking in a different account, I get the app vouchers, can still use a SNU in the other account, and be sure everything looks good, before cancelling the original.