Because there is no guarantee what a new owner will do with legacy products, what is the best way or practice to preserve old versions of Sonar?
 
I have really old Projects that I may want to get back to some day, if only to check to see what was done on them, and I figure keeping older versions around is smart for that purpose. If BandLab decides to radically change the Sonar business model (including deleting all the old update patches from their servers), if I download all the patches now to all the older stuff and keep the original CDs, will that be enough to ensure that I have backups of the old versions should I need them? Do the CDs phone home (I can't remember) to see whether one is a registered owner of the serial number and could this be turned off at their end, so that maybe keeping virtual machine backups of fulling patched versions is a better preservation method? What is your advice? Thanks. 

All versions after x3e require on line activation. They use CCC or the Offline activation code which both require contact with the server. Bandlab has promised to keep this going while they are in charge. 
 
All versions X3e and backwards do not need on line activation. Only the Serial and registration numbers so those will always be available until you no longer have a computer/OS that supports them.. 
 
Most of us survivalists have already downloaded everything on our "my Products" page and stored it away in a safe place. 
I made a copy/ paste notepad of all my serials too. 
 
If you have real old versions you can make ISO images of the DVD/s too. I use \Free ISO Creator\ 

A full-backup utility such as Acronis is your best insurance. Even if your whole computer goes up in smoke, it'll let you restore an image of the original drive(s). 
 
Even this is not 100% assurance, though. If your new computer has a different motherboard, network adaptor or disk drives, some software will not work until re-authorized. That includes a couple of Cakewalk products, such as Dimension Pro. Don't know about SONAR, but I suspect it'd be OK. If anyone has upgraded their motherboard without replacing disk drives, they'd be able to tell you if they had to re-authorize SONAR or not.

wlow
...if I download all the patches now to all the older stuff and keep the original CDs, will that be enough to ensure that I have backups of the old versions should I need them?...

 
Yes. I keep an archive of all of the installation files that I have ever downloaded, including Sonar patches. Over the years I have installed Sonar 7, X2 and X3 on several different platforms without any issues. Make sure that you have all the registration codes. If not, find them on your My Products page.