Yes you may amend number of headcounts in all of the modules anytime, if you are increasing the number then payment will need to be made first in order for it to take effect, if it's downgrade, no payment will be required. However, it is NOT recommended to DECREASE the number of headcounts in any plan as there will NOT be any refunds once the payment has been made. Although if you increase the number of headcounts in the plan, then the cost of the upgrade will be prorated accordingly.