Valet switch PHP version with these commands
Install PHP 5.6 and switch Valet to PHP 5.6
valet stop
brew unlink php71
brew install php56
brew install php56-mcrypt
brew link php56
valet start
Then to switch back to PHP 7.1 it is as simple as:
valet stop
brew unlink php56
brew link php71
valet start
Then to switch to PHP 5.6:
valet stop
brew unlink php71
brew link php56
valet start
I had to do
valet use php@7.4 --force
without it, it only changed the cli, and phpinfo in the browser showed 8.0