This change is primarily designed to tax Thais. Wealthy Thais have kept funds outside Thailand and earned income that is repatriated in the year following the year in which it was earned. This income has been tax free up until now.
It seems that many readers here do not understand that it is your duty as a citizen of the world to know where you are a tax resident. Anywhere you spend 180 days between Jan1 and Dec31, you are tax resident. It is now your duty to file tax in that country.
Also, if you pay tax on income in Canada, for example, but that income when reported in Thailand is taxed at a higher rate, then you report the tax paid in Canada as a deduction on your tax payable in Thailand, and pay the balance owing to the Thai tax authorities.