You're splitting mighty fine hairs. They will make some back end changes, if they do a decent job they will also do a bunch of testing and adjustments for the client too. If they didn't I would question their capability, anything in IT or development you wouldn't just bring up an older version of software and just run it and call it a day. You would run into a lot of problems doing that.
The main reason I asked that of you is to make sure I understand your position. From what I understand you think they will make NO changes to the client, and only changes to their server instances.
If thats what you truly think then I am sorry that won't be the case. They will test it, they will make adjustments to ensure the game runs well on a whole new host of operating systems and hardware. This is blizzard they had to do this for Starcraft remastered as well. They have standards.
This does not mean that they will change the game drastically. This is purely compatibility issues I am talking about.