If for whatever reason you need to downgrade your reseller plan, please pay careful attention to the following.
Firstly, you need to downgrade your own hosting plans, and reduce the total amount of "allocated resources", so that your not trying to fit 60GB worth of resources into a 20GB plan, as this will cause you to immediately be over-quota, and nothing will work properly whilst you are over your self inflicted lower quota.
Secondly, once you have reduced your plans, and adjusted your allocated resources (Used or not) then you can go ahead and downgrade your reseller hosting plan successfully.
It is very simple, but has to be done in the exact reverse order that your reseller plan started off.
If we see you order a downgrade where your monthly usage for either the current and/or last month would not fit into your proposed downgrade, we may contact you to discuss it or deny the downgrade order and remove it.
