This depends on whether it's minor revision or major revision (you provide me some information initially, then you opt to vary it). If I completely got to rewrite the document because things have changed on your side then yes, I will be able to invoice for it. If there are just a few minor changes here and there which will be completed in a few minutes, I don’t charge extra. I also don’t charge extra if the revision must be done thanks to my oversight or misunderstanding.