when they make changes there has always been a start date that has applied from both the date of application and date of arrival. The new rule starts from applications made from 1 March for Thai Pass and for arrival after 1 March .Previous versions have all worked this way both good and not so good .It is Thailand rules not what others expect
your Thai Pass if approved was originally an application, if you use that to enter Thailand you are governed by the rules as they are today ,including hotel requirements, insurance requirements and so on.The only way you can get in under new rules is to apply and arrive after they start .