When I switched a room only to a package a few years ago, they were able to apply my old deposit to the new reservation. (at least I think they did...I'd have to go back and check)
Keep in mind, if you switch to a package less than 45 days from check in, you will be required to pay the balance of your package at that time.
EDIT: I just checked, and my original deposit was refunded and then I was charged the $200 deposit.
So, if you really want to do this you'll have to make sure that you can cover the new deposit (and possibly the balance) without your original one night deposit. It can take a bit for the refund to post to your account.