After upgrading to Melodyne Studio 4 I can no longer register Essential with Sonar/CbyB on a new PC.

Celemony voids the license used to upgrade and provides 2 installs for the upgrade product. Do not use any Melodyne installers supplied with SONAR once Melodyne is registered or upgraded. Use the installers supplied in your Celemony account.

I had this too. Send an email to support@celemony.com and they will help you. In my case it was caused by too many Windows(10) updates. After every update the PC seems to get a different identification / fingerprint and the registering website told me that I had tried to activate Melodyne too often on different PCs...

I can't think of another software vendor that cripples their introductory version after you spend money to upgrade.

You need to read the instructions as this is very clear. Sure it's not my idea of a good way to run your copy protection, but it's how they have chosen to do so.
I had to e mail them twice now. Both times I had forgotten to un-register a PC before I upgraded Hard drives. But they ask no questions and send you a new serial # with in a day. 
 

komposer
 
I can't think of another software vendor that cripples their introductory version after you spend money to upgrade.

I have Sonar on my main music PC. Upgraded to Melodyne Full version and then found out that the Essential version on my home pc that I also have Sonar on, wouldn't work anymore. When I found out why I was PO'd. I thought the same as you.
 
But I have upgraded Melodyne everytime since. It's a good program. I learned to live with it.

It's pretty common that if you upgrade a program, you don't get to keep using the original program -- the one that qualified you for the upgrade. If, on the other hand, you had paid full price for a completely separate license, you would be able to keep them both.

The issue that I have mentioned to Celemony a few times is that people do use programs in different ways, so a "light weight" option would be nice for people who want to open an entire song in Melodyne. Editor 2 did this in 10 seconds, Studio 4 takes about a minute, but there is no option that I have seen to disable all of the "added goodies" Studio 4 offers. A simple "run in Editor mode" would suffice (for me).
 
I am not sure, however, if different versions of Melodyne can co-habitate on the same machine. I have seen people mentioning this, but the installations conflict with each other in some odd ways (both directories and registry entries). Melodyne Essential is included with more DAWs now, so it is easy to have multiple licenses.
 
Since the higher versions encompass the lower ones, a simple "mode" button would be nice to have.

mettelus
...Melodyne Essential is included with more DAWs now, so it is easy to have multiple licenses....

I've got a few ARA compatible DAW's (SPLAT, Studio One, Samplitude, MixCraft) which all came with a Melodyne essentials license. None of them forced me to use that license, and all of them required me to install Melodyne separately - just like Sonar did.
 
Every one of them automatically picked up my Melodyne Editor version which I upgraded from SPLAT's Essentials license.
 
M.

Yeah, I assumed that was highly probable... hence not cohabitating well. My experience demoing Studio 4 was a nightmare to Editor 2. A disc image was the only surefire way to back out of that demo.