There may be a provision as follows: "Change of occupation: If the insured be injured or contract sickness after having changed his occupation to one classified by the insurer as more hazardous than that stated in this policy or while doing for compensation anything pertaining to an occupation so classified, the insurer will pay only such portion of the indemnities provided in this policy as the premium paid would have purchased at the rates and within the limits fixed by the insurer for such more hazardous occupation. If the insured changes his occupation to one classified by the insurer as less hazardous than that stated in this policy, the insurer, upon receipt of proof of such change of occupation, will reduce the premium rate accordingly, and will return the excess pro rata unearned premium from the date of change of occupation or from the policy anniversary date immediately preceding receipt of such proof, whichever is the more recent. In applying this provision, the classification of occupational risk and the premium rates shall be such as have been last filed by the insurer prior to the occurrence of the loss for which the insurer is liable or prior to date of proof of change in occupation with the state official having supervision of insurance in the state where the insured resided at the time this policy was issued; but if such filing was not required, then the classification of occupational risk and the premium rates shall be those last made effective by the insurer in such state prior to the occurrence of the loss or prior to the date of proof of change in occupation."