In the past Megabonus promotions Gold and Plat elites have been able to call MR and get their promotion "changed". The change though is really a notation in their profile stating what promotion they desire. When the promotion is over, a rep will see the notation and manually add in the difference in points the elite would have earned if they were given their desired promo.

If you call and ask, as a Silver, you might be able to change or you might not. It can't hurt to call and ask though.