When you open an old project with the new Documenter version, you will be prompted to upgrade by creating a new project.
Normally, the upgrade is chosen, in which case a new project will be created with all your settings copied to the new project. The new project belongs to the new formatter version. Retain the old project for backup purposes.
The only time you would consider not upgrading to the new formatter version is when you have already entered many paragraphs of your own text to the generated document and do not want to repeat that in the new version. Because you have to generate the project from scratch after upgrading the project, your changes will not appear in the document until you re-add them. If your project does not contain manually-added content or if you do not mind re-adding the material, the upgrade is always recommended.
Should you choose not to upgrade, Documenter will continue working on your project just as the previous version did. Documenter supports several formatter versions, including the previous version allowing you to continue working on an old document without upgrading if you so choose.