Thank you very much for the instructions!
Did you solder the new WSON-8 chip directly on the pads meant for the original SOIC-8 chip?
And did you add any insulation to the center pad of the WSON-8 chip?
The AC2 HAP board has a pad for both SOP and WSON
For chips in the WSON package, the center pad is used to attach the chip to the board.
But it is quite possible to put the flash drive in a WSON case and on SOP pad.
The dimensions and pinout coincide there. But it is worth fixing the chip with glue.
I did this for HEX S. But this is another unfortunate story
Thank you very much for the info!
You used a hot air soldering station to solder the WSON-8 package, correct?
And you soldered also the center pad of WSON-8 package and did not use any glue in ac2 device, correct?
Yes, that's right.
Without a hot air soldering station, WSON is difficult to change. But it is possible.
Also socket adapter for WSON8 can be used, like this https://vi.aliexpress.com/item/1005005261973767.html. This will simplify soldering, but particular adapter from link doesn’t look robust and not sure is it reliable when board is shaked when transfering device. Maybe adapters with better quality and locking exists.
Thanks!
One more question - after replacing the flash (and copying all the data from the old flash) were the MAC addresses (of ethernet and wifi interfaces) and device model and serial number (visible in /system/routerboard/print) preserved?
Thank you very much!
Socket sounds like a huge improvement here!
It seems to me like the latch is safe enough, this other link has better pictures:
https://www.aliexpress.us/item/3256804602719717.html
IF (BIG IF) one could manage to drill two holes (1.1 and 0.9 mm) at the right spot on the board for the two plastic pins, the placement (and consequently the soldering) will be a breeze.
Or you can just cut off the plastic pins.
BTW soldering these low pin count SONs with a normal soldering iron is totally possible, if not necessary fun. And the cheap hot air stations go for under USD 100 and work completely fine. (Of course the USD 1000+ brand name ones look nicer 
)
Many people - especially at first - really like using a gel flux such as https://uk.farnell.com/chip-quik/smd291nl/flux-syringe-5cc/dp/1850220 (just as a reference, buy whatever is available locally)
Well, the plastic pins are very useful to keep the socket in place (though as hinted still the holes for them won’t be easy) while it is totally possible to solder the socket without the pins, it is not at all easy to solder it while keeping it aligned.
Personally I wouldn’t even think of attempting this kind of soldering without some means to hold the part in place, there is a very simple design for a “smd beak” here:
https://vpapanik.blogspot.com/2015/02/the-smd-beak.html?m=1
Well.. I’m an EE and neither me, nor my colleagues, nor the technicians that I worked with had any difficulty in soldering these things with just a normal soldering iron (Weller TCP / Hakko fx-888, etc.) and tweezers. (And with better ones like the Hakko fx-951 it’s much easier.)
I tried to look up a video on youtube showing how it’s done, and the first 5 or so hits that I found - while successfully attaching their chips in the end - did a really messy job of it: lot’s of bridging, cold joints, reheating everything several times, applying tons of unnecessary flux, bending/forcing leads etc.
I came here to call @jaclaz on his bull, but apparently he’s right and people do struggle with this. I’m also reconsidering my original comment suggesting to just solder the SON component directly. If people have this much trouble with normal SO and SSOP stuff, they’re unlikely to succeed with a SON.
Apparently? 
Your auto-correction must have messed with the “undoubtedly” you wanted to type. 
Okay, I couldn’t resist adding an example of how to do it correctly. Watch from 3:17
https://www.youtube.com/watch?v=PYIEP0un88o&list=PLtC8xXD7xz1hArt-i33vyaiLby3VJcMe6&index=3
The guy is an instructor with top notch equipment including proper non-ocular (stereoscopic) magnification.
I'm also interested where this data is preserved and which data is stored in the OTP part.
Is it possible to grep the serial number or stuff like the Mac addresses from the Chip's image? I yes did not had any time to put my flash reader clamp onto one of my cAP AC's
AFAICT there is not OTP (used by router OS) on these flashes.
The issue with the license comes from a sort of GUID that is in the chip and that is used to calculate the license number, when you replace the chip this GUID obviously changes and your license does not match anymore.
If you are going to peek into the flash dump, you will find useful this thread:
http://forum.mikrotik.com/t/filesystem-forensic-image/181321/1
The OTP area can be read and written. But the serial number, on the basis of which the Software-ID is generated, is in the area that is written at the factory. And it cannot be changed. That is, each flash drive has a unique, unchangeable number. This procedure can only be done with an HDD disk. But this is quite difficult.
After cloning the firmware the only problem is the license. MAC addresses remain the same. They are (only one:) ) available in the open form in the dump.
In the HARD section of the flash drive image, there is a parameter RB_ID_FLASH_INFO 0x03 with a size of 14 bytes. It seems to contain the size. After replacing the flash drive with a 32 MB one, try checking how much space is available and how much ROS will display.
If you have already disassembled the AC2 and are soldering it, it makes sense to re-solder the RAM chip to increase the size to the maximum 256, with the corresponding correction of the RAM size in the flash drive image RB_ID_MEMORY_SIZE 0x0D.
I usually reinstall the OS using NetInstall. After that, the free memory is displayed correctly. What RAM chips should I use? Is there a list or specific models? I have an AC2 on hand, and I want to try replacing the RAM chip.
I didn't change it myself. Here is the information from the 4018 processor specifications
DRAM memory
JEDEC standard DDR3L SDRAM, Up to 256 MB, Supports 16-bit DDR interface, 533 MHz clock rate; 1066 MHz data rate
They say they took chips from old SO-DIMM DDR3L 2Gb, which have 8 chips on the board.
Can I use W25Q512JVEIM? Is it necessary to program the flash or can I install a blank one and load it using Netinstall?
Of course not. Flash drive contains a Boot Loader that allows you to use NetInstall. You need to create a copy of your Flash drive or use any dump from a similar device (but you'll lose the serial number and MAC address).