The Philippines has already amended the Constitution singly in the past: when Congress was changed from a unicameral to a bicameral assembly; and when Parity was passed to allow foreigners to own property in the Philippines. We’ve done it before, we can do it again. Best of all, it would be cheaper. Congress can discuss and pass the proposed amendment during its regular session and submit it to the people for ratification at the same time as the regular elections. The same thing can be done for other amendments.
A constituent assembly to change the whole Constitution is dangerous, what with the quality, or lack of it, of the present members of Congress. The same legislators will pass the proposed amendments on the piecemeal basis I propose, true, but then the debates would be closely watched by the people.
A constitutional convention would be expensive. We would have to elect delegates to the convention, each of whom would have an office and staff, and the convention itself can go on and on, having no time limit.
In the question whether we (1) Do Nothing or (2) Mount a sweeping Constituional Reform initiative, the earlier -- Do Nothing -- still has options within it; initiatives that are less likely be turned into a circus and can proceed in an environment of sobriety, mature engagement of the public, and a focus on ideas rather than on people.