Daylite 6 overwrites previous versions of Daylite when it is installed. To test Daylite 6 Self-Serve you should install Daylite 6 and Daylite Server 6 on a separate Mac. Any new or edited information in Daylite after updating to version 6, will not be available if you revert back to Daylite 5.
To test Daylite Cloud, you will also need to install Daylite on a separate Mac, and you should use a secondary email address. This is because the email address is tied to the account and is non-transferable. An easy way to do this is if you have a Gmail account, you can add "+" and addition characters to the end of the Gmail address. For example, if your Gmail address is firstname.lastname@example.org, you can use email@example.com to test Daylite Cloud and still receive these emails at firstname.lastname@example.org.