Legacy Pay As You Go

I lost my gold status long ago and was only rewarded another year standby when I called in.... After that I had to add $10 every 3 months. However it seems adding an entire $100 holds it for a year. (Gold or not) All good if You make use of the service enough to require refill. To be honest, I have zero trouble with the phone working except only ONE time the entire T-Mobile network crashed. I use the dollar a day plan but I was curious if I could find the actual details on how the plan works today example if I add $100 then $10, will it nag me in 3 months again? Understanding they wouldn't want to loose money but there is obviously not enough grandfathered plans to effect the company budget.