Apologies, but could you explain this one?
rather than paying monthly, you can buy pre-paid vouchers for 3/6/12/24/36 months of Xbox Live.
eg 12 months of core is £33 on cdkeys right now.
apply this to your account.
then buy one month of xbox ultimate for £10 and apply it to your account.
it converts your 12 months of core to ultimate at a 3:2 ratio, ie you get 8 months of ultimate in exchange for your 12months core + 1 month ultimate.