Can we change the hours billed on invoices without changing the invoice amounts?

No. You need to delete the invoice before you can change the hours to be billed. You can also void it to keep track of the action.

Note: If you use Progress Billing, you can attach more time (and expense) entries to an existing invoice. This adjusts the hours on all the entries linked to the invoice (using Write-Up/Down) without changing the invoice amount.