Selecting all posts displayed, they click on "Revert to draft", and are given the bad news.
You cannot modify more than 50 entries at once.This is annoying - but does not have to be terminal, to whatever project was underway.
You can only revert 50 posts at a time, using the Posts menu.
The Posts menu will only change 50 posts in one action.
Given this limitation, change the page size, to show 50 posts / display page. Once that's done, you can "Select all" in a display page, and "Revert to draft" - and revert all 50 posts, displayed.
You can make a record of the posts, before you revert them to draft.
Don't revert to draft, with no chance of knowing what you had. Make a copy of the Blogger generated sitemap, and save it in a safe place. Having some way to research post URLs and publish dates may be useful.
With the sitemap saved safely, do what you need to do.
I normally display 100 posts / display page. The Posts menu will only process 50 posts / change.
So, change the Posts display page size, to 50.
Now, we only display 50 posts / display page.
Use the "select all" box (which, on mouse over, shows "Select all comments"), to select all posts in the display page.
And, with all displayed posts selected, hit "Revert to draft".
With more than 50 posts involved, page to the next display, hit "Select all" again, and repeat.
Republishing will require post by post changes - so be prepared.
When you re publish, it's going to be another project. Unless you were very lucky, some posts will have been reverted in a different month from when they were originally published - and the re publishing will likely not be in the same month.
You're either going to have to change each post publish date back, one by one, to the original - or you're going to have to add custom redirects, one by one, to resolve the "404 Not Found" errors, caused by the new URLs.
This is where the copied sitemap becomes useful - so you can, orderly plan the re publishing effort. Or, plan to use Search Console regularly, watching for "404 Not Found" errors caused by the re publishing.