It depends upon the type of changes that are being made to the webpage.
If you are only modifying the layout or display, or making other revisions that don't substantially change the content, then you do not have to keep a copy of old webpages.
If the changes to the webpage include revising, adding or removing substantive information like policies or procedures, requirements, deadlines, program or course descriptions, then the site must be retained permanently as a historical or “Archival” record. Other sites that contain research data should be kept for 6 years after the end of the research study under which the data was created.