changing the subdivision of a budget

Hello, @cath-whoisyourne. I can see how you want to modify an existing budget according to your business needs. 

Please know that changing the Subdivide by option for an existing budget isn't possible in QuickBooks Online (QBO). As a workaround, you will need to create a new budget and choose Subdivide by class instead of "customer'. 

You can refer to this article for your guide: Create budgets in QuickBooks Online

If you want to assign classes to transactions, you can do so by following the instructions given in this article: Track your transactions by class in QuickBooks Online

I'll be around if you have more questions about budgets in QuickBooks Online. Leave your reply below, and I'll surely get back to you.

So I have to delete my budget and start again because I did one thing wrong?

I can provide you with clarifications about the subdivision of budgets in QuickBooks Online, cath-whoisyourne.

Currently, changing budgets that are subdivided by customers can't be switched to another subdivision type. A workaround for this is to create a new budget with Subdivision by class. I'd be glad to walk you through the process:
 

1. Click the Gear icon.
2. On the Tools column, select Budgeting.
3. Click the Create Budget button.
4. Choose Subdivided.
5. Click the drop-down menu, then choose Subdivided by class.
    

I've added this article if you want to track your finances within the program: Run reports in QuickBooks Online.

Please reply or comment on this thread if you have additional concerns about budgets in QuickBooks Online. The Community is always here for you.