Thanks for getting back to us, @schmidtfs.

Let me help you further find a suitable app for changing classes to Products and Services in bulk in QuickBooks Online (QBO).

While I can’t suggest a specific third-party app, I can suggest a couple of keywords you can type in the Find apps page search field. In the search field, type in the keywords like Inventory or Inventory Management. 

After this, read the app reviews to assess its features, product quality, and performance. That would help them choose the right one fit for their needs. I've attached a screenshot for visual reference.

As a future guide, let me add this article to get helpful insights for your client's inventory data: Use reports to see your sales and inventory status.

You're always welcome in the Community if you have further concerns about finding a suitable third-party app for classifying your client's products and services. We'll do our best to assist. Take care.