I just received my first Bitcoin today, and I noticed that a new bech31 address has been added to my wallet, while the previous bech31 address is no longer visible.
There is no bech31 address. What you are talking about is bech32. They are addresses that start with bc1.
What will happen to my Bitcoin if I decide to transfer some sats to the former address?
You will receive the fund and there's nothing to worry about. You have numerous addresses all generated from a single seed phrase and you can use any of them.
Take note that all bitcoin transactions are publicly recorded on the blockchain and anyone in the world can see them. That's why your wallet generates a new address for you, so that you can protect your privacy if you are going to receive another bitcoin transaction.