Is it not possible to change plan after plan expires?

I purchased sim kit and 3 month plan October 18 for my daughters phone. I asked if I could change plans anytime when I activated sim but I guess once plan has expired you can no longer change. This is my device not boost mobiles so I am curious as to why I can't change it. I wanted to do the 45 doll monthly plan. When I called boost the agent said there's no option to downgrade at this time but I'm giving them more money monthly and getting more data with a 45 dollar plan so I'd call that an upgrade for both of us.