As a 56-year-old US citizen with a retirement visa, if I have cash in a US account (with taxes paid), can prove it was there before Thai law changed, and plan to move it to Thailand later, what steps can I take to avoid taxation in Thailand?

Please obtain a bank statement showing the account balances at 31st December 2023. This cash can then be potentially remitted to Thailand without any tax implications if it was pre-2024. Always put on the remittances ‘pre2024 savings.

Keep good records as you can be audited for up to 10 years.