Yes I figured that 800 MHz FSB CPUs weren't compatible.
That's why I tried the pentium T2390 because it is 533 MHz (2 core pentium Merom) but the machine doesn't run stable enough because of power draw or other things.

When I mentionned lighter OS it wasnt the name of the OS itself just saying I would like a light dristro.

I did my research before trying to upgrade but as I said I already know the percentage of success would be low but I was OK taking the risk.

When this lady came to the association I already told her when I realized it was a single...

The CPU pins are in very good conditions (no missing pins or bent, no contamination with foreign material on the pins side).

The T2390 is showing some residue or mark of something I couldnt get rid of with isopropyl alcohol and rubbing gently with towel paper on the top of the CPU after it was installed on the socket.

About the BIOS, good point. In videos where I see people upgrading the CPU of similar model Acer Extensa 4220 they don't have the same version as me sometimes it is V1.30 or V1.31.
So the laptop I have apparently got V1.33 so newer version.
...

Hello,
I wanted to share my experience trying to upgrade the CPU of an old Acer Extensa 4220.
This is a laptop of an old lady who I am trying to upgrade what I can and refurbish the rest. Some people are very possessive and want to keep their stuff as much as possible.

So the laptop specs originaly is :
CPU : Celeron 550 2GHz single core (533MHz FSB 31W TDP)
RAM : 2 GB DDR2-5400s (667MHz)
Bios : Phoenix V1.33
CPU socket : mPGA479M

The computer was using Win10 but was completely unusable so I installed Zorin 17 Core.
It's usable...