Premium to Family and change of payment method?

[faulknerm]

I'm currently a Premium member, have been for 12 months, and pay via my mobile phone bill (charge to mobile, rather than any kind of deal with the network themselves). Looking to switch to the Family plan so that 2 family members I live with can make use of the service too, however the mobile payment method isn't available - not an issue, happy to pay by card.

Just concerned though that I've already paid for a full month via my phone bill, so don't want to pay for half of the same month again by card - if I upgrade to family today will this happen? 

Am I best waiting until the day before my mobile payment usually gets processed to change to Family and pay by card, then at worst I'm only losing one day?

Cheers,