My client is billed for a service that was upgraded or downgraded during the month - why is it still billing for the higher value?

This depends on the setup of your product and the billing principle selected for this product. It is also important to keep in mind that if a change was made to the provisioning of a product or service on the source system and the Source Collector has already imported data for the day (If your Source Collectors are scheduled to run daily) it would not immediately pick up the change. 

If the clients' invoice is still open / failed, you can reverse this invoice to then update the product settings or update the Record ID of the billing records before reprocessing this invoice. The changes will then reflect on the newly processed invoice.

Read more on docs.varibill.com for more information on how your invoice line item is calculated or how you can maintain your billing data.