Yes, you can change your plan per line anytime during the bill cycle. If you switch plans mid cycle, any data previously used will count toward the data limit and your bill cycle dates will remain the same. The data usage tracker will not be reset when the plan is switched (data used will be cumulative). All data used during the cycle will be charged based on the new plan. You will be charged taxes and fees based on whichever plan is in effect at the time of billing.