There's a way. 

1) First, search for System App Remover (its a paid app so..),

2) Then install it, open it and go to System Apps. Look for Launcher.apk and uninstall it.

Now, you should be able to install LPP just like normal, but without the mods like you seen in the LauncherPro that comes with Elegant Blue. If you want the mods, follow this lengthy step. *deep breath*

1) Uninstall your Launcher Pro Plus if you had just installed it

2) Look for your LPP apk file, rename it to Launcher.apk

3) Use rootexplorer (a paid app, again ) and copy paste the Launcher.apk to system/app (make sure you mount r/w [option on top right of screen in rootexplorer])

4) Set the permissions of launcher.apk to 2/1/1 (take a look at the other system/app files as reference, tap and hold one of them and press Permissions, and look at what is ticked. Follow it on the Launcher.apk you just pasted)

5) Reboot

6) Now download the metamorph file from the Elegent Blues thread, place the zip file in your memory card.

7) Now download Metamorph from the Market

8) Open metamorph, press Start Checks [allow superuser] (if it fails, go again. It should be able to after a few tries).

9) Now go Unzip/Extract New Theme, look for the zip file you downloaded (should be LauncherProSummerBlues.zip)

10) After done extracting, go Pick Existing Theme, select the metamorph file, and let it install and reboot.

11) Enjoy.