Awhile back I reinstalled Sonar Professional on a new computer. However, I never got around to reinstalling Melodyne b/c I rarely use it. I decided to install it today, but the set-up/registration system keeps telling me I'm using a product key that's already registered. (That's true, because I did register the product on my older computer). So I'm unable to use Melodyne now, it seems. I'm wondering if this problem is caused by the recent Cakewalk/BandLab shake-up. Any suggestions/potential fixes? Thanks for the help...

I went through something similar 4 months ago, where having not used their product ever, I got around to updating and trying to use their product.  My Sonar X3 Producer would not let me.  All I remember right now is that I ended up writing their support team and letting them know that I had license to their single voice product because it was bundled in my Sonar X3 Producer when I bought it (4 years ago).  I included the Serial Number to the product that was licensed to me in my Email to them, and be sure to use the same Email that they have on file for you, which I believe is important.  (I have all those numbers saved in a separate document on a separate drive).

They responded by sending me a link to download an updated version 4.1.1 from what initially was 2.x, and gave me a new Serial Number, too. They made some improvements and they are still honoring their commitment to the version I had gotten originally.  And you can install that file (4.1.1) on any of your other computers, the same way Cakewalk let's you install on any number of PC's, so long as you are only using one at a time.

Hi. I have had that happen to me, either after a Win update, or fresh Win install.
I've resorted to Celemony's support, and they have been quick to respond.
All the best.

Did you try to remove your old computer at their site?
Log in to your account, and remove the old computer. That will allow for a new activation.
http://www.celemony.com/en/melodyne/what-is-melodyne
 
 

Melodyne is installed via custom installers from the Ceremony site. For a new machine, log into your Ceremony account and click on your products and download the installer. You can also delete machines there if you exceeded your limit.

As said you have to remove the old computer from your registration before you nuke it. 
So if you still have the old computer go in and un register it from Celemony.
If not you can Email support and beg for forgiveness, they were very nice about this to me.  

Many thanks for the advice, everyone. I don't recall ever creating a Celemony account (pretty sure everything happened through Sonar), but I'll see if I can dig something up. If not, I'll just contact the Celemony support team and hope they're understanding.  Thanks!

Well if you have ever used Melodyne in Sonar, you would have had to register an account with Celemony.  All your Cakewalk account provided was a code that you needed to activate on the Celemony site to claim your license.  The Melodyne essential license was not locked to Sonar.