So - it is a year or two out from completing your book and you want to make it a hardcover too, or want to make some simple sounding change such as changing the price on the back cover. At this point the completed project has been archived on a backup drive.
Changes to completed projects, after the pdf files have been delivered and the project is paid for, may be treated as new project.
Should the author wish substantial text changes (More than 5 or 6 words) after page layout begins, the author must submit new complete and proofed chapter(s) for replacement. Replacing a chapter or replacing multiple chapters will billed at the current hourly rate but should rarely cost more than the page layout base cost.
Revisions to covers, adding a second cover style or making a cover change - When a project has been put to bed, it is a bit more work. To access and correctly modify an archived project has to include bringing that project back online, refreshing memory of the project and it's pieces, and then making a new modified cover with the changes. If the change is a change of price. That will require a new bar code for the new price.
If the new work is to create another page size or cover type, such as adding a hard cover, the new format needs a new ISBN. Both the new cover and the interior needs to be modified to reflect the correct ISBN for the format.
Work done to make changes to covers or inside after the project is completed will be billed at the current hourly rate with a minimum of one hour.