Under the PF & Miscellaneous Act - I think section 12 - an employer is prohibited from decreasing the salary/ wages of an employee. In other words, if you decrease the basic such that the total contribution of the employers share and the employees share gets decreased, then that will cause the PF officials to raise an inquiry with the employer.
To reduce the basic salary, it is better to give increase on other salary components like travel allowance, etc but at the same time keeping the basic constant. That way the net salary will increase but the basic will remain the same, and over subsequent increases the basic will finally be reduced in ration to the net salary.