Could an upgraded Harrier replace the F-35B?

If the JSF programme failed, could existing Harriers be developed into a fifth-generation STOVL aircraft?

Not realistically; the changes needed would amount to a new aircraft, which is why joining the F-35B programme made more sense.

A hypothetical failure of the Joint Strike Fighter programme would leave no easy path to an incremental Harrier replacement. Britain had examined the problem and would need a nearly new aircraft to approach the required capability. Joining the F-35B programme therefore made more sense than rebuilding a legacy airframe around a new generation of demands.