This is not Adobe Customer Support, and we are not Adobe employees. This is the kind of question which you'll need to get answered by calling Adobe Customer Service. There should be a contact phone for that on the Adobe website.
I guess it would work just like if you wanted to transfer ID from one computer to another... you would need to deactivate it first (under the Help menu).
I think the person who posted is talking about multi-user licenses which work differently than the individual licenses we buy for our the software. But I'm not familiar with the detail of how those licenses work, which I why I referred him to Adobe Customer Service.
I have found Adobe Customer Service to be less than helpful when I have had to use it in the past, including conflicting answers from different support personnel. I was hoping that someone on this forum had done this. I guess they have not, so I will bite the bullet and contact ACS.
As a practical matter, I don't think it matters too much. There's a certain amount of the "honor system" in volume licensing keys, and the real point is that you paid for the number of seats you are using. If the program doesn't complain about it, I would not spend too-too much time worrying about it.
It's not clear to me if your Y includes your X or not; that is, was X=100 and you added 200 seats and now X=100 and Y=200? Or X=100 and Y=300?
Volume licensing key activations aren't any different from regular license key activations, you can still uninstall (or deactivate, I believe) and install with a new key.
But your "terminal server" environment may complicate the situation if Y does not include X. You could have two seperate installs of ID, or you could have ID installed with either X or Y but not both.
I am not a lawyer and this is not legal advice.