If planning ahead, could plant a SHA-256 message somewhere in your feed, then if you ever need to use it you can post the clear message on your new npub and point to the old SHA, to show that you were the one that posted the message on the old npub
Wasn't really something I looked into since it was extremely unexpected. Also up to believe official GrapheneOS platforms would suffice in announcing my key change. I dont want to spread fear by people misunderstanding a personal and isolated incident as a GrapheneOS one.