Nostr Web Client

Your "A" wallet is probably a standard "pay to pub" wallet.

Your new multisig will be "pay to script" where the multisig condition will be enforced.

You need to transfer all old funds to the multi-sig wallet.

Do you know the difference between utxo, address and wallet?

Hmmm I think I know what they are..

Utxo is a balance in a public key.

And public keys are made from private key.

Finally wallet is a channel that made you can see the balance in private key or public key....