Thanks for taking interest of the Class feature in QuickBooks Online, Matt21.
Currently, we don't have an option to assign multiple classes to the existing transactions at once. You'll have to open each transaction and assign the class one at a time. You can use this link for reference: Create and manage classes in QuickBooks Online.
You might also want to utilize the migration tool suggested by Fiat Lux -ASIA as an alternative or workaround.
We'll be right here if you have additional queries in managing the classes and your transactions. Have a great day!