Switch to another prepaid plan not possible

Hi,

my wife has the 5gb prepaid plan for $30 per month. She wants to switch to the 3-month 8gb plan for $33 a month. The plan is not available in her profile and also the customer service on the phone or the store was not able to switch her over. The person on the phone recommended to go to the store and the person in the store recommended to enter it as a new order. Unfortunately, that does not work either, as the number cannot be transferred (I guess because it is with ATT already).

Any other ideas what we could try?

Thanks!