For monthly plans
If you add a billable collaborator to a workspace on a monthly plan, you won't be charged until the next monthly billing date (which is typically the same day of the month as when the workspace was first upgraded). On that day, you'll be charged a prorated amount for the period of the previous month during which the collaborator had billable access to the workspace, as well as for the forthcoming month.
If you remove a billable collaborator from a workspace on a monthly plan, on the next monthly billing date you will receive a prorated refund of Airtable Credits for the days that the collaborator did not have billable access to the upgraded workspace.
For annual plans
If you add a billable collaborator to a workspace on an annual plan, you will initially be charged for the full remaining annual commitment for that collaborator. If you remove a billable collaborator from a workspace on annual plan, you will be granted a refund in Airtable credits prorated to the day the collaborator was removed.
Adding a collaborator:
Three months after upgrading a workspace to an annual plus plan ($120/collaborator/year), you add a billable collaborator to the workspace. You would then be charged an additional $90, for the new collaborator's access for the remaining nine months of the annual commitment.
Removing a collaborator:
Ten months after upgrading a workspace to an annual pro plan ($240/billable collaborator/year), you remove one billable collaborator from the workspace. You would then receive a prorated refund of $40 of Airtable Credits for annual commitment's remaining two months
These examples are simplified for the sake of clarity - any of these charges or refunds would be prorated to the day any billable collaborators are added or removed.