I've written a few times about template conversion, and migration between Classic and Layouts, and migration between Layouts and Classic too. Reverting is easy, but I'd never tried reverting from Layouts to Classic, and back again.
So can you go back and forth, without losing template content?
When you revert, you get a popup advice
If you revert to your Classic template, you will still be able to recover your current design.
It's not lying to you. The path back isn't obvious, but you can get back. Until you elect to publish the blog by FTP or SFTP, that is.
Here's my test blog, http://nitecruzrtestnew.blogspot.com/, which has lots of odd and complex objects arranged in it. I just reverted it to use a Classic template, as a test.
When I view my blog, it indeed has a Classic template.
So you go back to the Dashboard, click on the Template link. From the "Edit HTML" tab, you may have to select "Customize Design". You'll now have the advice to "Give your blog a whole new look!" (again!). Hit the orange button, and you get to "Select a new template". Pick the same template that you had before you reverted, and it should come back with the Layouts Template blog just as it was previously.
Click on the orange button, to recover the template back to Layouts. If you don't see a button, or if the button doesn't do anything, you may have the New Blogger 2008 Upgrade button problem.