Backup / copy to other device / disk what is important , next install normally, only diffs. = do not format root partition , swap + /boot can be formated if needed.
This should install onto and replace the system / world files but not user files /home/elvis ones but everything else would likely be replaced. Then afterwards update / reemerge specific programs you might need. Might even be able to update / add to /boot if not formating during this not sure.
Worked for me might work for you. Use at own risk / lossing data.
Hope this helps. Also check to make sure you have enough space to do this , dvd 3.4 versions uses at full install 12gb space + then you need space for temp files and other stuff.
minm. needed space total = 20gb => " /boot 128mb - swap ? 2gb - rest of 20gb remaining disk space for root partition =