Hi bigwobble,
Your billing date will be monthly and needs to stay so - as well as being the date on which the subscription is taken, it's also when the usage reset script runs so if we moved it on 2 months you'd have to stretch one month's usage across 3 months.
I can confirm however that there's a 100% discount against your subscription for the next 2 months as promised, so you won't be charged either this month or next.
I hope that reassures you,