The amount of a residential ground rent may be changed on demand of either the obligor or obligee at the end of five years from the date of the agreement, and every five years thereafter, by giving notice to the other party by certified mail or overnight delivery using a commercial service or the United States Postal Service between 90 and 60 days prior to such fifth anniversary. Unless the parties agree otherwise, such change in ground rent shall not exceed the percentage change for the preceding three years in the Average Consumer Price Index for all items, all urban consumers (CPI-U), as published by the Bureau of Labor Statistics of the U.S. Department of Labor or such other instrument or agency of the United States or of the Commonwealth as may be designated by the General Assembly. The first of such years shall constitute the base year.