Rollover Data expires after one billing cycle, when you change your plan (including changing your data amount), or when your account or eligible plan is ended. You say that Rollover Data expires when plan changes are made.

How long does rollover data last?

Simply put, networks that offer data rollover allow you to take any unused data from one month and use it the following month.

What happened rollover data?

Your Rollover Data expires after 1 bill period or with a plan change, like changing your plan's data amount. You can't: Redeem unused Rollover Data for cash or credit.