https://www.cmegroup.com/tools-information/lookups/advisories/ser/files/SER-7425.pdf I have read this document, they didn't change notional value (contract size), just price increments (tick size and tick value). Multiplier is the same. However in Russel 2000 example above, they actually changed multiplier and with that contract size from $118300 to $59150.
Another thing that can happen (not quite the same as the multiplier change but still causes problems) is a specification change. For example, changing the notional coupon or possible years to maturity on the deliverable basket for a bond future. This has happened a few times. GAT