Change our Mailing/Billing Address Ahead of Time?

TaRDy thought this was worth mentioning said
4

I am moving to a new address today in about 8 hours and have been wondering for the past month, what do I do if a Fuku comes up and I need to buy something fast?

How can I change my default billing and shipping addresses and ensure my payment will go through correctly ahead of time?

I know I could just email halp@meh.com but I figured this might be useful to others to see the response.

Yes, I could just buy any product leading up to the next fuku, but let's assume that is the last resort option.