You will need to contact Telstra to get the answer. Without knowing what plan you were on, plus whether you signed up for a trade-in, it's difficult to figure out. Plus because you've already chosen one option and want to change it muddies the waters a bit. Normally when you upgrade early, you have to pay out the value remaining on the phone on your next bill. If it was a lease phone, you get charged the market value of the phone. There are too many options to cover here.
Send a message using the My Telstra app on your mobile (blue icon, bottom right of Get Help screen). If the bot doesn't understand the question, type "Speak to a consultant" and you should be transferred to the queue to chat with a human. It may take a few hours to get an initial response.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.
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Setup direct debit