It would surprise me if that would ever happen apart from at bush meetings due to a change of circumstance
AR.120(b) . .If a rider, after having been declared is found to have accepted a ride for which he is overweight, such a rider may be punished and another rider may be substituted at the allotted weight,or nearer to the allotted weight
Apprentices not being able to claim their full allowance may be a different story, but I am sure that they are required to declare their riding weight with the control body