As you intend to keep the Snapshot for longer than VMware's best practice, I would suggest you Clone the VM instead.
Yeah thats what my gut is telling me all right. I'm just wondering if I clone it will that change the OS's SID.
Yes, it will create a new SID, but that's shouldn't be a problem providing you have a local account with local admin rights and a domain account with rights to add servers to domain.
When you have finished with the Clone you must power it off
Power on original server - use the local account to logon
Change the server to be part of workgroup - restart
Logon with the local account - Join the server to the Domain (provide domain account and password) Restart
That should be it.
Cloning will only change the OS's SID if you choose to do Guest Customization (the cloning wizard has a dedicated step for that).
If you choose not to do it, from the OS's perspective, a snapshot or a clone are the same thing.