PowerShell for removing service application association

I have created a Managed metadata service application named MMD1.

It's associated with 2 web applications, say webapp1, webapp2. Now I am creating a new MMD called MMD2. I would like to remove the MMD1 association with webapp1 and webapp2 and associate it with MMD2.

I am trying to do this with PowerShell. But I do not know what API to use. Help is appreciated.

$serviceapplicationname = "MMD1" 
$spapp = Get-SPServiceApplication -Name $serviceapplicationname 
$spguid = $sapp.id
$webapp1 = Get-SPWebApplication http://webapp1