Whenever you change your billing settings (either billing method or hourly rate) or approval settings on your project, you can decide from which date onward you want these changes to be applied.
You can even apply your current settings further in the past by choosing the appropriate time frame.
The default mode is that your changes will be applied from now on, but you are able to change the date by clicking on the link below.
Then the popup will appear where you can select to which time logs these new settings should apply to.
Here you can select either "From the beginning" or a custom date.
When you click 'Save', all time logs on this project will get their billing rates re-calculated.