MSI architecture change from x64 to x86 does not remove PuppetLabs folders in Program Files due to File System Redirection with 32bit msiexec.
It also doesn't remove the older path - this is what remains after install of x86 AFTER x64: C:\Program Files\Puppet Labs\Puppet\bin;C:\Program Files (x86)\Puppet Labs\Puppet\bin and it should be C:\Program Files (x86)\Puppet Labs\Puppet\bin.
A where puppet evaluates to the program files folder first, which could have some very bad inconsistent results if this is not cleaned up.
If you downgrade (true downgrade) from 3.7.0 x64 -> less than 3.7.0 installer, you are subject to what the older installer can handle. We should document that if you want to go down, you should uninstall first and then reinstall the older version.