Below is an answer to basically the same question (I copied this from another thread), however this person was trying to get back to Open Home rather than LP. The main thing here though is to clear defaults on Home. Once you do that, the next time you press the home button you'll be prompted as to which launcher you'd like to use.
Downloading Home Switcher is a darn fine suggestion to, it basically does this for you.
-----------------------------------------------------------------------------------------
Go to settings, then applications. Click on "manage applications" and then bring up the menu to see the "filter." Filter so that you see "all." Scroll down to "home" and click it.
When you do that, you should see a "launch by default" that says it's set to launch by default with certain situations. Click "Clear defaults." Click your home button, and you should be able to re-set Open Home as your default. Hope this helps.