First, the cuts must have Cabinet approval, which has not yet happened (though it seems almost certain it will) and then a Royal Decree. Asked how long that would take, an official from the Phuket RO said she did not know.
Until this happens, personal income tax will continue to be assessed at the rates in force last year.
The RO official also reminded foreigners that if they spend more than 180 days in a tax year in Thailand, they are liable to pay personal income tax, not only on earnings in Thailand but also on earnings abroad, unless those earnings are made in one of the 56 countries that have tax treaties with Thailand.
People from a tax treaty country must submit a certificate from that country to the RO, confirming that they have paid tax.