Just insure your boot environment for the beta is set to "keep" (System Settings>Boot>Click the kebab menu>Keep) before you switch. Moving to the nightlies will create a new boot environment(s).
From there, just boot back in to it to go back to the beta. You can choose environments at startup and also choose which boot environment to boot in to from the GUI. I believe you can even export your config from the nightly and re-import it to the beta if you make a lot of changes but I haven't tested that recently.