If I move a member account from Invoicing Unit “A” mid-month to unit “B” how does the spend show up?

When you move a member account from one Invoicing Unit to another mid-month, the behavior of how the spend is shown depends on the type of invoice being generated.

For Anniversary Invoices, if an account is moved from Invoicing Unit "A" to Invoicing Unit "B" during the middle of the month, the entire month's usage and spend for that account will be reflected in the invoice for Invoicing Unit "B". This means that even if the account was part of Unit "A" for half the month, all of its charges for the full month will appear in Unit "B"'s invoice.

It's important to note that changes to Invoice Units can be made at any time during the month. The system will apply these changes to the entire billing period for Anniversary Invoices, regardless of when the change was made.

This approach ensures that the full month's costs for an account are kept together, which can simplify accounting and cost allocation processes. However, it also means that organizations need to be aware that moving an account mid-month will result in all of that account's charges for the month appearing under the new Invoicing Unit, rather than being split between the old and new units.

For accurate cost tracking and reporting, it's advisable to plan account moves between Invoicing Units carefully, considering the impact on billing and cost allocation.
Sources
Configuring your AWS Invoices using Invoice Configuration | AWS Cloud Financial Management