I'd prefer space by space, or page tree by page tree.
We tend to have several docs in one space, so it would be useful to do one document at a time (i.e., a page tree).
Site-wide honestly sounds scary to me because generally, we have a lot of formatting problems to work out after converting anything to the new editor. Converting hundreds of docs to the new editor at once would mean thousands of pages that would need manual editing to repair bad formatting.
For additional context, we have to export our documents to PDF, often at a moment's notice, so the potential for a bottleneck is huge. If a document needs to be exported immediately but has been converted to the new editor as a result of a site-wide conversion, and we didn't have time to fix the formatting, it would be a huge problem.
Space by space or page tree by page tree allows a greater level of control, while offering a massive improvement to page by page.
Hey everyone, the space option has been rolled out to all tenants and we are in the middle of rolling out the site option which should be available to all tenants by early August.
As a result we will close this ticket as done.
Thanks for your patience on this feature and check our our community post! https://community.atlassian.com/t5/Confluence-articles/Migrators-you-can-now-bulk-convert-legacy-editor-pages-to-the/ba-p/2432943