If the import is completed, then you can use our EXPORT and MODIFY feature to update the transactions which are already sent to QB.
When I import invoices, the class field is available to map, but after the import, the class information does not import. This is important for us to be able to import items line by line with different classes, due quantity of lines, it would not be feasible to go into QBO and amend each invoice individually. EXPORT and MODIFY doesn't work, the class still does not import.
There are 2 types of classes.
If you want to get class details at Transaction Level, you need to map it to Class field.
If you want to get class details at Line Level, you need to map it to Product/Service Class field.