This is the bane of my existence and why I hate the way a lot of these companies are setup with their licensing...
 
I'm going to be reinstalling Windows 10 when I get my new computer built which means I'm also going to have to reinstall all my plugins and reregister them. The problem with this is that some companies require me to unregister their plugins first before I do this or I "lose" the key. I'm pretty sure Melodyne does this and I think I actually moved my final key over to iLok because of it, I'll need to check on that.
 
This kind of crap really drives me nuts though. Companies that use a type of DRM that limits the amount of installs you can do need to cease to exist.
 
What other companies require you to unregister or have limited number of reinstall keys do you know of? The only ones I can think of is Best Service which I can request more and Spectrasonics which I'm not sure what their policy is.

Waves licenses should be sent to the cloud during the transition
and then returned ex post facto...unless they are on a USB drive.

If you already have 2 authorizations on anything toontrack you will need to delete 
one in order to re-authorize.  

Same with XLN products. But can be done at any time.  

All your Fxpansion products should be deauthorized and then re-authorized
when they are reinstalled.

iLok licenses are no worry.

I'm sure there are others....