So I got a Pro license key and want to upgrade from the free license. I went ahead and clicked the “Enter Serial Number”->Reactivate buttons. Every time I click the “Reactivate” button Unity instantly freezes. I can’t see the cursor over the window and it doesn’t appear to be doing anything.
I tried reinstalling Unity to just start fresh, but I guess it didn’t remove the key in the uninstall process… Is there any other way to switch to my new Pro key?
I’m having the same issue. Is there a file somewhere that’s remembering the license I can delete?
Your only option is to contact Unity 
lame… ok well hopefully the support people get back to me shortly 
Well, you could try manual activation.
Having the exact same problem. And about manual activation, I cant even get to this point: I’m only presented with a “Reactivate” and"Cancel" buttons, and if I click Reactivate, Unity will just hang.
The only thing that solved it was I needed to reinstall windows ( for another reason). I guess I lucked out… Or had really bad luck that made me go to the extreme.
On windows 7 its a security issue, or at least it was for me. Go to User Account Control Settings and lower the notify setting to the lowest possible. You will then have to restart your computer. Restart open Unity and there will be no problem reactivating/ activating. Afterwards you can turn the security setting back up.
Call me blind, but I cant find this notify setting 
Could you please give me some more directions on where I can access it? Thanks in advance!
Here are two ways to get to it:
1:Type in User Account Control Settings in the start menu search. Set notify to never notify.
2: Control Panel/ User Accounts/ Change User Account Control Settings. Set notify to never notify.
For security reasons after you have reactivated Unity you are probably going to want to set the notify setting back to the default.
Thank you Brian!
Mine copy of Unity also froze up when I tried to activate it.
What helped me was running Unity as an Administrator (rightclick on Unity shortcut → run as Administrator).
Note that it might take a minute or so before a browser(page) comes up, but in the end it does work.
Thought I share my solution to. Just in case
Thanks wicked, this worked perfectly for me.