Apple Silicon: port all Homebrew packages under /usr/local/opt/ to /opt/homebrew
According to https://docs.brew.sh/Installation, /usr/local is for Intel binaries, /opt/homebrew for ARM. So you may want to reinstall instead of just migrating.
You can dump the list of currently installed packages with brew bundle dump. To duplicate your current installation, you'll need to

run brew bundle dump to create a Brewfile

install ARM Homebrew into /opt/homebrew by following the instructions in https://docs.brew.sh/Installation
cd /opt
sudo mkdir -p homebrew
sudo chown -R $(whoami) homebrew
curl -L https://github.com/Homebrew/brew/tarball/master |\
    tar xz --strip 1 -C homebrew


make sure that the ARM version is at the beginning of your PATH
PATH=/opt/homebrew/bin:$PATH


Reapply the brew file created in step one
hash -d brew
brew bundle install --file /path/to/Brewfile`


Optionally run brew analytics off to turn of the tracking/analytics gathering done be Homebrew (https://docs.brew.sh/Analytics)


PS: Some formulae don't seem to work on ARM yet