By default your monthly fee will stay the same. Once the MaxDSL rollout is complete you will be able to select the speed you want via the portal and if you want to move to a different package and a different monthly fee, again you will be able to do that via the portal. An upgrade (increase in the monthly subscription) would apply immediately and be done at no cost, and a downgrade subject to an admin fee and apply at your next billing date.
I'm currently on 1mb Premier (£29.99 per month) if i was to 'downgrade' now to 2mb + 5GB (£20.99 per month) then am i right in thinking i would not have to pay to change to the £21.99 30GB MaxDSL option when it's available? And by doing it this way i'll be able to get my maximum speeds sooner, by not having to wait until the next billing date?
Moving between the current and new products and what it will cost has not been determined fully yet. If you did it now it would cost £24.99 I think (speed change and change from premier -> lite) but what costs are involved to move to the MaxDSL based premier account has not been defined yet.