Use a standalone ALF file... It's good forever. That's what I've done since they started the network/on-demand licensing silly shit way back when. We have machines that sit behind One Way Transfers - see the patent for one here:
https://patents.google.com/patent/US8250235B2/en . So we have no access to the internet. With the ALF file you have control of you disaster recovery. Just maintain backups of the ALF file and Full Installer of the version your ALF is good for (full installers are available on their Downloads section). Back the shit up of those on multiple media and you're good to go on any machine you may buy.
Even tho the Altium guys hate me I have to say that there is three things good about Altium:
1. a simple licensing scheme using a simple license file. Just like it used to be. Control over your own destiny. This is in-line with the "edict" found in the Fenwick and West document (the guys that did the first shrink wrap for Apple back in 1975) - see page 13 of the PDF
http://www.fenwick.com/FenwickDocuments/Patent_Licensing.pdf Did you ever wonder why Adobe and Autodesk killed licensing? This is why... Conversely, if you RENT software the user is at risk of not being able to open his own Intellectual Property at a later date WITHOUT interaction with the rentor. This creates some interesting legal issues that have yet to be fully hashed out in precedents of various courts. One of the things that you have to consider when using a restrictive license like Autodesk or even a cookie jar type license from Solidworks.
You could end up with a bunch of files of YOUR DESIGNS in a proprietary format that you can no longer access. Recall your designs are your property. Having to rely on or get permission from a third party to exploit your work is questionable at best. Be interesting to see this tested in court. Autodesk vs. Vernor was just the beginning (even tho Vernor may have been violating copyright laws - not sure).
2. you can have multiple versions installed side-by-side. In fact, if you use custom install directory names you can have point versions installed in parallel - ask any seasoned user on the Altium forum - it's the way to make sure a new version with bugs doesn't hose your ability to get stuff done
3. Earlier versions of Altium will for the most part open files saved in later versions. Even the Host Dave mentioned this in response to one of my rants -
https://www.eevblog.com/forum/altium/a365/msg3189118/#msg3189118 and when he worked there. I will say that some of what might be in an AD21 file can get funky in something like AD15. But an example is the stuff created in the new Layer Stack Manger - things like Rules that are based on impedance set in the LSM Impedance tabs - converts to plain generic rules with the correct values in previous versions like AD17. Pretty cool.