Hi pauloz,
As always there are plenty of ways to skin a cat
I'd suggest as cleanest and fastest solution a new install:
1. Perform your intended testing with a live USB stick. See whether:
* the friend likes it
* all the hardware is working as expected
* the system is running as expected
2. Check the current system for customized settings and apps. Collect as much information as possible of what s/he expects to see and use after the swap.
3. Backup all personal data and configuration, mainly the home directory and maybe also /etc.
[If you want to go the extra mile and have enough external disk space and time you could even backup the whole system, for example via clonezilla or systemback. Just in case s/he changes his mind, or something goes terrible wrong. You could even clone it to a portable disk (if on hand) to start it from there.]
4. Install LL2.4 and transfer all the personal data back and install the additional software required.
My 2c, let's see what others suggest
If you need more detailed instructions please let us know.