If, after going to For Invoicing, you see different amount instead of the correct amount on the timesheets (or even just a portion of them), it is likely that the timesheets are still using the original case rates.
To correct this, simply re-save the rates on the case - this will load the current values and the amounts on the timesheets will automatically correct.
Just follow these steps:
Discard the existing invoice
Go to the case
Resave the rates
Then you can recreate the invoice - the amounts will be correct.