The information logged should be similar to the previous long-form invoices that the system generated so that it would be possible to tell what the customer had, and what they got after the upgrade. If logged to file, (or database for that matter), it would probably be ok to store the data in JSON format.
I would lean toward storing in a database so that it can be easily referenced from Blesta (possibly an option on the service to view the history in the future), and so that we can store a longer history (We may not want records to be rotated out so quickly).
What information does this aim to log exactly? I think this should be broken into two parts that don't necessarily need to be done at the same time: