Virgin Mobile systems are unable to correct mistakes made

I requested my package be downgraded to a cheaper package with more airtime and data because I cannot afford the heavy bill every month. I was promised it would be effective 1 April - It was not downgraded as promised. I queried it before the end of April and it was not corrected a second month. I will pay for my old package at the end of May again. I've been told the system cannot be corrected once packages have been loaded. I was offered one months free service on the new package as compensation but in my understanding the offer is half the value of the money Virgin Mobile's mistake has cost me. I have therefore requested a cancellation of my service on the 5th of May and now I am going to have to pay for a 3rd month at the end of June in lieu of a month notice to be given on cancellation. 3 months at the higher rate all because Virgin Mobile couldn't put me on the package I requested on the 1st of April as requested. I think that the least Virgin Mobile could do is backdate my cancellation to the 30th of April in lieu of service not delivered and release me on the 30th of May and not load the old package again on the 1st of June.