since you are not switching the computer on which you want to run Houdini, you don't really need to return the licenses.
Apprentcie HD users have a license account available online and can follow the following steps to install the old Houdini HD Apprentice license on the new operating system on the same computer:
- log in to the license website:
http://service.sidefx.com [
service.sidefx.com] using your email
address and your password
- in the “Manage Licenses” link, find the license you want to install and click
on the corresponding license ID link in the first column. This will bring up a
window that contains the license keys for your machine (SERVER) and your
Houdini HD Apprentice (LICENSE)
- launch the Houdini License Administrator (hkey) on your machine, and choose
File > Manually Enter Keys…
- copy the SERVER line you obtained in the second step and paste it in the
first field
- copy the LICENSE line you obtained in the second step and paste it in the
second field. You can refer to the example provided in the Houdini Manual Key
Installation Dialog.
- for Apprentice HD you will need to copy-paste all three LICENSE keys
(Houdini-NonCommercial, Render-NonCommercial, and Apprentice-HD), but
since each of these keys needs the same SERVER key, you need to copy-paste
only one SERVER key. Thus you will end up with four fields filled up in that
dialog window: first one is the SERVER key and the last three are the LICENSE
keys.
- press the Accept button and the license will be installed on your computer
Just to explain a bit more, when you initially purchased Houdini HD, you were given an entitlement, which you were able to redeem as a license on any machine of your choosing. Once you redeemed that entitlement as a license, the license became bound to that computer; or in other words you converted the entitlement to Houdini HD into a Houdini HD license for that computer. This is the reason why you got the message that you don't have any entitlements when you tried to install a license (i.e., redeem an entitlement). Instead, you now have the licenses themselves.
One thing to point out is, that since the license is bound to your computer, if in the future you wish to transfer the license from one computer to another, you will need to return your license by right-clicking on the license in the Houdini License Administrator and choosing “Return License”. This will convert it back into an entitlement which you then will be able to redeem on another machine. However, if you are not switching the physical computer, you do not really need to return the license. You can just reinstall the existing license by following the steps above.