Generally if you are on the maintenance program, we suggest that when you upgrade an account you request a new set of installation keys to match the release they are using, user count, and any packages they may have. While these are normally sent out with when you place an order for some form of upgrade, in the case of CCP we don't know if or when you will upgrade the account so you will need to request them.
These keys will allow you re-install should it ever be necessary, even in case when the internet may be down or our servers offline for maintenance.
Now normally, when upgrading, we suggest you run the online registration on the existing PxPlus before installing the new release and then share the activation license file. On Windows, if using the default installation path names, sharing of the activation keys is automatic. For Linux you have to manually use the Linux 'ln' command to share the file or use an environment to point to a shared file.
As for your question about system crashes, for the past few releases the system has had built in detection to see if you are running from a restored backup. If so it will confirm that you want to move the installation to the new system and make an online request for the keys accordingly. This only works if you attempt to run from a restored backup and you can only do this once so be sure your new system is stable otherwise you may need to have us reset the keys for you.
Once the keys are updated it will flag the old system for automatic key deletion should you ever recover the old server and attempt to use it to run the original PxPlus install.
Lastly, it should be mentioned that in PxPlus 2020 (v17) you can use V13 (or older) installation keys and the system will advise you the keys are too old, however if you are attached to the Internet it will provide you the option to register online in order to get your current keys.
Hopefully this answers you questions on re-activation and how to recover from a system failure.