If any rights and benefits of an employee (e.g., annual leave, sick leave, etc.) will be varied depending on his/her employment duration, we should specify such benefits clearly in an employment agreement since the first signing date. The employer must provide those rights and benefits to its employee anyway.
The advantages of this approach are as follows:
(i) both employer and employee understand their own rights and obligations very well since the first day of employment;
(ii) there will be no misinterpretation, misunderstanding and forgetfulness of such rights and benefits; and
(iii) both employer and employee do not have to amend the employment agreement again when time passes and triggers new rights and benefits of the employee.
If you do not agree with my suggestion, you can consult this matter with Khun Apichai so as to check which option he prefers. By the way, I do not think that putting progressive rights of the employee about the employee’s annual leave and/or sick leave in the employment agreement will be much concern or difficulty.