IEAKs are generally useful for upgrading, however, if it doesn't work just right you can run into big headaches.

There is a solution, albeit a difficult one.

InCtrl5 is a utility that tracks changes made when you run an install. This will allow you to actually create your own install packages and/or update specific files that need to be updates from ie5.5 to ie6 or ie6 to ie5.5. Simply create a "good" installation of ie5.5 and run inctrl5, then install ie6 over it and run inctrl5 again. This will create a record of all the changes that were made. To go backwards, you can verify that all the files that were changed to version 6 are back to version 5.5 and all the registry entries are back to the old way as well.

Here is where you can get the utility.

http://www.pcmag.com/article/0,2997,s=400&a=4589,00.asp

Brian