The OSFullName is the guest OS name as obtained through the VMware Tools.
That is done inside the guest OS, and is what the guest OS returns.
What you see under GuestId is the guest OS name as you specified it when you defined the VM.
In fact you can pick any of the accepted values for the GuestId, and still install a completely different OS as the guest OS.
The OSFullName, provided you have VMware Tools installed, will display the OS name as reported by the guest OS.
The GuestId you can change, the OSFullName you can't.
Take this example, a test VM which was configured for CentOS 6, and we can easily change it ti display Windows 7 in the Web Client.
Get-VM-NameTestVM|SelectName,@{N='GuestID';E={$_.Guest.ConfiguredGuestId}}
Get-VM-NameTestVM|Set-VM-GuestIdwindows7Guest-Confirm:$false|Out-Null
Get-VM-NameTestVM|SelectName,@{N='GuestID';E={$_.Guest.ConfiguredGuestId}}
This results in
In conclusion, you can not change the OSFullName, but you can change the GuestID