Stock Minecraft servers can't do this, but Bukkit has support for running an arbitrary number of worlds. If you aren't already running Bukkit, you'll need to install it and set it up to use your existing world. (Be sure to make keep a clean backup of your world files somewhere safe in case of catastrophe!)
Once you have Bukkit running, the MultiVerse plugin (which also requires the transitional PermissionsBukkit plugin until Bukkit's own Permissions API is complete) will give you access to Bukkit's multi-worlds capabilities with commands and configuration files.
Once you have multi-worlds tested out and working properly, you will have to:
- Wait for Minecraft to update to 1.8. (I know, the suspense is killing me too!)
- Wait for Bukkit to update to a 1.8-compatible build. Because of the obfuscation in Mojang's code, the Bukkit team always needs a number of days to unravel the new server code, add Bukkit's changes, test, and release an update.
- Create a new 1.8 world. Probably the easiest way is to log in as an admin or op on the newly-updated Bukkit server and issue a simple MultiVerse
/mvcreate MYNEWWORLDNAME NORMAL command. Alternatives (such as the
mvimport command) exist and are covered in the MultiVerse documentation.
MultiVerse will then give you the tools for making warp portals, setting permissions on who can go and do what where, and all that fun stuff.