When I am reconciling banking and change the "Category or Match" field, it changes all the other transactions after that date to the same category.

Thanks for sharing your concern here in the Community, @darla1512. I've got you covered.

When QuickBooks seems to botch categories, the Enable suggested categorization option may be activated. You can deactivate this setting on the For review tab. I'll guide you on how:
 

1. Go to the Banking menu.
2. In the For review tab, select the mini Gear icon or settings.
3. Uncheck the Enable suggested categorization.

Here are some articles that you can refer to learn how to categorize and reconcile transactions in QBO:

• Categorize and match online bank transactions in QuickBooks Online 
• Reconcile an account.

I'm only a post away if you have additional questions about managing your QuickBooks Online transactions. Stay safe, and have a good one!