Most adult children put this off because every opening feels wrong.
If it does not go well
Some parents will not engage, no matter how you approach it. They may shut down, change the subject, or get upset. That is common, and it is not the end of it.
Name it and step back. “I understand. I'll leave it for now. I just want you to know I'm here when you're ready.”
Ask when they would feel ready. Let them name the time, so it stays on their terms.
Bring it up again later. One conversation rarely settles this. A calmer day often goes better.
Keep your own notes. Write down what you already know about their wishes, accounts, and documents.
Document your own wishes too. It takes the focus off them and shows this is something everyone does.