It's entirely feasible to download and begin using the update with your current data. However, it is safer to run the update using a duplicate copy of your data.

Regardless of whether you run the update with a copy of your data, the new implementation of recognizing sales tax is not totally backward compatible with invoices posted from Corona 3.9 and prior. The difference is in the description field of the sales tax line items of the invoices; prior to 3.9.1, Corona enters the customer's name, the new implementation enters the sales tax name and rate.

The sales tax reporting depends on the sales taxes and rates to be present instead of customer names. The sales tax report uses the description field to subtotal the various taxes and rates. The total sales tax on the report is unaffected, but each customer name will show a subtotaled amount. Corona cannot fix this automatically, so each invoice of the current year should be reopened and updated if you require the sales tax report to properly subtotal the taxes and rates. The image below further describes this issue and what to do to recall and post each invoice.