Change customer group after first order

[TurkeyFish]

PS 1.6.1.24

I'm using a module to set a minimum order threshold for new customers. When they place their first order I want them moved from the New Customers Group to the Customers group, where the threshold will no longer apply for subsequent orders.

How can I change a customer group like this? Seems like it should be easy with a little code? I suppose I could do a cron job database query nightly to update their group, but I was hoping to use a function and hook.