I decided to replace motherboard (Socket 771) to motherboard with Socket 775 on WIndows 7 (Ultimate, not activated yet) PC. And it was successful so i will write details:

(When handling with motherboard like adding/removing cables and cards, always turn off the switch on the back of the power source.)

-- Adding socket 771 CPU into socket 775 motherboard --

1. On new 775 socket motherboard i adjusted the plastic CPU socket (using knife) so my 771 socket CPU fits the 775 socket. It has to be done carefuly not to damage any metal parts that are very near.

2. On the back of the CPU (Xeon E5430) i seen there already is tiny reduction that seems like one shown on picture of the 771 to 775 socket tutorial - https://www.delidded.com/lga-771-to-775-adapter/4/
So i not needed reduction i bought.

The CPU FAN of 771 socket fits 775 in my case. The thermal paste may be needed, but the old paste that was on CPU was still liquid and i had no other, though i seen on ark.intel.com (https://ark.intel.com/products/33081/Intel-Xeon-Processor-E5430-12M-Cache-2-66-GHz-1333-MHz-FSB-) that my CPU max. operating temperature should be 67C and in WIndows, i see average temperature on all cores is around 75C and during load even 85-90C which seems wrong if motherboard not detect wrong values while i am using "incompatible" CPU.

I removed CMOS battery from motherboard for a minute to reset BIOS.

-- Connecting motherboard & startup --

After connecting main power cable, there was secondary power cable from power sourcem but new motherboard has 4 PIN secondary power socket.
In the motherboard manual i seen i had to use PINS 3,4 and 7,8

At first computer not stated, because i have not used Reset SW connector from the PC panel power button to appropriate jumper/socket on the motherboard.

Once it started, CPU fan was spinning, all good, i had to be immediately hitting "Del" key to enter bios (which was resetted upon removing CMOS battery) and then set back things i wanted, for example i advanced peripherals enable USB keyboard.

-- starting MS Windows --

Then Window started in low resolution mode and after login Windows started detecting various hardware and installing drivers. It took like 30 minutes on slow HDD. But it not installed ethernet/internet device. I had to open Device manager in MS Windows and under "Other devices" right click devices there and try to update drivers for it. It detected ethernet device and installed it. After i modified screen resolution and PC restart, all seems good like before motherboard replacing for different type.

I also used software Snappy driver installer origin to update drivers for me. I like this SW.

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There are tutorials on how to fix Windows after replacing motherboard, but i have not used these:
https://www.dowdandassociates.com/blog/content/howto-repair-windows-7-install-after-replacing-motherboard/
http://www.tomshardware.co.uk/answers/id-3847134/windows-start-hardware-change-motherboard-processor.html
https://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/repair-windows-7-after-replace-motherboard-cpu/