When i tried activating online it wont accept the code on my old account. Created a new one with different email , still wouldnt accept it. Opened unity 5.4 with my old account on it, went to help/manage license and in there it accepted it and the background changed to black instantly. Also after restarting unity it wont say unity personal any more, its just unity now and the about page says i have pro on all devices on the bottom right corner. So seems like it works with old accounts too.
I get an error message when I add the serial number for the 4 months of Pro - "This serial cannot be managed from the Online Store, please contact sales if you wish to make changes to it.". I guess I have to contact "Sales" at Unity. I used a new account to add Unity Pro code from Oculus.
So you have to enter the code into the Unity editor. It won't work on the website. So open Unity, click Help -> Manage License -> Activate New License -> Paste the code in the Pro Edition text box -> click Next. It should work on existing accounts fine, at least it did for me.