"This Nitro Pro serial has already been upgraded". Nope.

[Larry Corsa 0]

When trying to upgrade, I get a dialog like:

Enter your current product serial number to check your discount eligibility. We'll check what kind of upgrade discount you're eligible for, then guide you from there.
xxxxxx-xxxxxx-xxxxxx        [Check Eligibility]
[Where's my serial number?]
Thank you for your submission. This Nitro Pro serial has already been upgraded and you are currently operating on our most recent version, Nitro Pro 13.

But I am still running Nitro Pro 11, or was... somewhere in the process I was prompted to download Pro 13, which removed Pro 11 (including my stamps, thank goodness for backups).

I can't make sense of this.  Am I eligible for Pro 13?  How do I activate, or even find upgrade cost??

[AllainU 0]

Hello @Larry Corsa,

Thank you for reaching out to us through our Community Forums!

In line with the private message I sent you, looks like we need to uninstall the current version of Nitro Pro on your computer and install Nitro Pro 13. We then activate it using your serial number. Kindly follow these steps:
 
1. Uninstall Nitro Pro from your computer by going to Control Panel > Programs > Programs and Features then reboot the machine.

2. Please clear all your downloads folder or delete all downloaded Nitro installers. This step is important because we noticed that if you are saving all the installers and you download them in the same folder (like the downloads folder), the EXE can pick up any MSI installer in that folder and you may pick-up one of the older builds instead of the newly downloaded package.

3. Download your Nitro Pro 13 using these offline link/s:

x32 http://install.nitropdf.com/professional_13192356/en/retail/nitro_pro13_ba_x86.msi 
x64 http://install.nitropdf.com/professional_13192356/en/retail/nitro_pro13_ba_x64.msi

If you are unsure if your machine is using 32 or 64-bit, please refer to this link: https://kb.gonitro.com/knowledgebase/#/search/64/000003790 

4. After installation, you can activate by opening Nitro Pro, clicking on the Help tab > About Nitro Pro > Activation and entering new serial number.

Thank you for choosing Nitro and I hope this helps.