Changing the Homedirectory path in our active directory

Hi I am totally lost maybe someone can shine some light on this...

what i am trying to accomplish: Our Home directory is currently mapped to this path like this  \\MainServer\Bob.jones  For our users
What I am looking to do is change the Path to include the domain name after the server name like: \\MainServer.MyDomain\Bob.Jones by replacing the original
server name with the sever name and domain name. then adding after it the what ever appears after the 12 character 

Here is what i have been trying to use 

Get-ADUser -SearchBase ‘OU=Test,DC=mydomain,DC=com’ -LDAPFilter "(&(objectCategory=person)(objectClass=user)(!userAccountControl:1.2.840.113556.1.4.803:=2)(homeDirectory=*))" -Properties HomeDirectory | ForEach-Object Set-ADUser -Replace @{homeDirectory="\\MainServer.MyDomain.com\s($_.homeDirectory.substring(12))"}

On the left side of the pipe i am trying to limit the selection to only users that have an existing path, and the account is active. that part seems to work fine. 
the right side is a mess (maybe a left side too )

I would really appreciate any help i can get

Thanks 
Tom