WARNING!!! After the switch neither Manjaro nor Arch community will be supporting you. No guarantee. What are you doing is your own risk! Make a backup first!

Pacman config

First of all you will need to download a new mirrorlist

sudo wget -O /etc/pacman.d/mirrorlist https://www.archlinux.org/mirrorlist/all/

then edit your new mirrorlist and remove any mentions of manjaro* from /etc/pacman.conf

sudo rm /etc/lsb-release

Reinstall pacman-mirrorlist

sudo pacman -Syyu pacman-mirrorlist

If you are faced with errors due to broken dependencies- remove the issue package before. (in my case it was linux41-nvidia* to be removed).

Updating pacman

sudo pacman -R manjaro-system
sudo pacman -Syyu pacman

Note that if you start getting "warning ... SyncFirst in section 'options' not recognised" you will need to comment that line out.

sudo sed -i 's/^SyncFirst/#&/' /etc/pacman.conf

Removing and reinstalling packages

sudo pacman -Rsn $(pacman -Qsq manjaro-)
sudo pacman -Syu alsa-utils alsa-firmware lib32-alsa-plugins 
sudo pacman -Rsn pulseaudio pulseaudio-alsa
sudo pacman -Sy pulseaudio pulseaudio-alsa 
sudo pacman -Rsn lib32-mesa lib32-nvidia-utils libgl lib32-libgl

If you don't have libgl* packages installed go ahead to the next step

sudo pacman -Sy libgl lib32-libgl --force

You will want to choose an appropriate packages to be installed (in my case it was nvidia-related ones). You will want to agree to remove mhwd. If during removing there is an error regarding mhwd-tui remove it first.

sudo pacman -Rsn $(pacman -Qsq mhwd)

Setting up Arch installation

sudo pacman -S grub

You will want to merge /etc/default/grub.pacnew with the existing one (especially replacing GRUB_DISTRIBUTOR and commenting out color options).

sudo sed -i 's/^Manjaro/^Arch/' /etc/issue

Edit /etc/release to have it look like this:

NAME="Arch Linux"
ID=arch
PRETTY_NAME="Arch Linux"
ANSI_COLOR="0;36"
HOME_URL="https://www.archlinux.org/";
SUPPORT_URL="https://bbs.archlinux.org/";
BUG_REPORT_URL="https://bugs.archlinux.org/";

Copy the file to /etc/os-release and remove /etc/manjaro-release.

Installing kernel and graphics

sudo pacman -Sy nvidia-340xx

Replace it with your driver.

sudo pacman -Sy linux linux-api-headers linux-headers linux-firmware

Final steps

1. Boot to grub and choose Arch Linux
2. Remove remained Manjaro Linux kernels
3. Reinstall packages that are marked as newer than available.

Done!

Congratulations!