The partial decrease of preference shares is doable. The legal restriction on this issue is that the registered capital of the company may not be reduced to less than one-fourth of its total amount (Section 1125 of the Civil and Commercial Code).
For example, at present, the registered capital of SCAL is THB 150 million. Therefore, the maximum capital reduction each time would be up to 75% (or by decreasing THB 112.50 million [1,125,000 shares] out of THB 150 million [1,500,000 shares]).