If I add data this month, can I remove that data next month?

[EvoHarley]

if I upgrade my plan this month can I down grade next month without penalty

[Weth · Accepted solution]

Yes.  Make sure you time it for the first day of your new billing month.  Also make this month retroactive to the first day of the current billing month otherwise you will get prorated data and risk of overage you are trying to prevent. 

[lewisr13]

Just to help clarify a bit more. You can upgrade at anytime and back date it to the beginning of your billing period. Always back date an upgrade. You should downgrade on the first day of your billing cycle, you cannot backdate a downgrade. Just familiarize yourself with how month in advance billing works so you don't end up confused and back here claiming they overcharged you.