I am currently on unlimited text, unlimited minutes and 3GB data plan. I want to downgrade to another plan My question is when will be the new plan effective? Immediately or the start of next billing month?
That depends on how you change it. If the change is done via Self-Serve it's basically immediate, maybe a minute or twelve. If you call customer service you can have an immediate change too but you can have it change at 12:00AM of the first day of your next billing cycle or even select a specific date (the change will still happen at 12:00AM of said date).
However, there's a $10 fee for changes made with customer service.