HT4627: iTunes Store: Associating a device or computer to your Apple IDLearn about iTunes Store: Associating a device or computer to your Apple ID
You have to delete the original application in order to change the apple ID login. The original application contains a key that is locked to that specific account to prevent theft. All purchases made through Apple are locked to the ID it has been purchased from to prevent theft including applications. In order to update, they must be removed and repurchased with the new login.
The apps are assigned to that Apple ID and there is nothing you can do to change that. You could choose to download them again with the new Apple ID. Any paid apps will need to be purchased again.
Hope that helps.