Not an easy task IMHO.
But there may be a way other than backing up/copying the files and configuration edits you want to keep.
Thankfully, there is an archive of all versions including the upgrade files past and present here:
http://sourceforge.net/project/showfile ... e_id=89552
You may be able to travel up the versions using the archived upgrades until you finally arrive at the lastest version.
Sounds messy but might work for you rather than the even messier alternatives.
Before you start this just make a copy of your whole xampp directory
(usually by dragging it holding down the Ctrl key
so that if the upgrading goes belly up you only have to rename the copy back to xampp and you are back to where you started.
Remember that when there has been changes to the server's configuration files in new versions then it usually involves uninstalling the out of date versions and installing the new version afresh but this will be seen in the upgrade notes if it applies.
So it would be no use making a back up of those server configuration files to copy them over the newer versions but from your backups you can edit the new version conf files to add the changes you made in the older version - can you follow that as it is an important point when having to install again.
HTH and good luck with it.
Take a look at the new DeskTopXampp
launch control for XAMPP
(DTX.exe) posted by ridgewood
http://community.apachefriends.org/f/viewtopi ... 967#103967
Available at Ridgewood's new
DTX web site http://zedfiles.com/DTX/
Also available here: http://nat32.com/dtx/
I highly recommend DTX.