Hello @delawaresealcoat,
As this time, any changes you made in QuickBooks won't affect any of your existing data. With this, you'll have to manually update the sales tax selection you've chosen for your existing estimates.
Lastly, here's a few articles you can read for ideas specially for your customers and sales transactions: Income and Expenses for QuickBooks Online.
Keep me posted in the comments if you have any other questions. I'll be here to lend a hand.