Okay thanks!
So account "A" and multi-sig wallet are totally different at the beginning.
Did I understand correctly?
Discussion
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....
I'm not sure LOL