The trick is to make sure your top level parent folder is visible in the Folders panel - right clicking folders and Show Parent Folder. Then, if the drive letter changes, you just need to right click that top level parent folder.
For example, let's say that what you see in Folders is 2011, 2012, 2013 and then monthly and daily subfolders. If the E drive changes to F, you'd have to right click each folder and point it to F:/2011 etc. Imagine these 2011, 2012, 2013 are actually subfolders of a Pictures folder, which is shown in Explorer and not in Lightroom. If you now right click 2011, Show Parent Folder, you'll see Pictures in the Folders panel. When the drive changes again, you just have to right click the Pictures folder and update it.
Also, can you set a network share address. So the E drive would be the network share //Photos. You'd have to update the top level folders as above, pointing to the network share instead of E, and you'd have to import new files from the share rather than E, but you'd then be immune to the letter changing.