Normally, the process is:

• install the packages for the desktop environment
• log out (not just locking the screen)
• find a dropdown or cogwheel where you can select the other desktop environment
• log in

Having said that, I don’t know what you mean with “graceful”. Desktop environments may involve lots of packages, which may create configuration files in your home directory or get auto-started in your other DEs, so it can be messy.
Something minimal, like LXQt or the various window managers, isn’t going to cause much of a mess, though.

I guess, creating a second user with a separate home-directory, like the other person suggested, would isolate that potential mess…

Just add a new user

Create a different user for each desktop environment.
Put all the users in a group that has full write access to a shared folder you use for your files.

Linux is a multi-user system, use that.