Actually, there's a shortcut to this!
Just get your original message,
Then
remove the address from the message (
copy this):
-----BEGIN BITCOIN SIGNED MESSAGE-----
I'm Lulucrypto on Bitcointalk. And normally, I'm Luluwebmaster. I sign this message at 9 Aug 2019.
-----BEGIN SIGNATURE-----
IAIAMSyjMV62EttLm3HltwmQK0HEchc80OfXKJGPEo1pIvq/st/kgWvLmREfByk3/TSbdrWLmfzoExivGSxzTOo=
-----END BITCOIN SIGNED MESSAGE-----
And paste to
https://brainwalletx.github.io/#verifyIt'll automatically recognize the legacy address used for that signed message.
Because either way, the message was verified using the address: 1NtMnD5BQrRvVeHDk4HXaGvXiVkUuTjhXf, not bc1q7qgn8zw75n26hd60a8ay42482mukdjrdv3cyp7.
But both can be derived from the same prv key, so there wont be a serious problem with future verification.
The only difference is: those extra steps are some kind of proof that the result legacy address was based from your SegWit address.
Thanks for sharing this.