Service / promotion matter

If a person get senior scale after 12 years and in non posting of higher post performs the duties and responsibility of higher post for 7 years. and after completion of 5 years RR rules changed and 100 % pramotional post convert into 50% LDCE (limited departmental examination) where 3 years experience required . he qualified that exam but due to less seats and merit he has not selected. does 17 years experenece and 5 yeares extra duty oand expereance of higher post will be null and wide ? and employee will retire on same post