I am not an authority on Philippine Constitutions, but as I see it, a shift from a Presidential form of government to a Parliamentary form of government might entail changes or the total scrapping of 9 out of 20 articles of the 1987 Philippine Constitution.
Article V: Suffrage
Article VI: Legislative
Article VII: Executive Department
Article VIII: Judiciary
Article IX: Constitutional Commissions
Article X: Local Government
Article XI: Accountability of Public Officers
Article XVII: Amendments or Revisions
When talking about a shift to a parliamentary form of government, I think one might actually mean coming up with a constitution that is vastly different from the one we have presently.
Can you imagine politicians (and they will be all mostly politicians) carrying on in either a Constitutional Convention or Constituent Assembly in a bid to basically tear down the structures from which they derive their power and influence?
Can there be so many Filipino politicians who are presently thinking of nothing but the common good?