I would think that would be possible, but I am not sure how it would respond when the database is an old version. The first thing you usually have to do after upgrading is update the database to the new version of SWPDM. You can't even access the admin tool until that is done.
I would test it and see what happens. Backup the database on the production system. Install SW2018 on your test server, create a new vault with the same name as the production vault. Go into the SQL management studio, delete the vault database and restore the backup with the same name.
Then run the database upgrade tool. Copy the vault archive over from the archive server to where it is located on the test environment and see if you can access it. It seems to me that it should work without any issues but I have not tried it. I have only ever installed the same version on the test machine and then upgraded to the new version.
We just did this very thing last weekend. We had to migrate to a new server since we were still using Windows Server 2008 SP2. SWX 2018 requires Windows Server 2012 or higher. We contacted our VAR and they helped set up the environment and moved PDM to the new server. We even moved the PDM Archive Server over with all the files (140,000+) . During the client upgrade we removed the old vault view pointing to the old server and added a new vault view to the new server. With our VARs help it was very painless.
The only prerequisite we did was set up the server first (obviously) and install the SolidNetWork License Manager on the new server then pointed all of our 2016 machines to it. This made the upgrade a little smoother on the clients since they were already pointing that way. We are now in the process of running the File Version Update process.
One thing I did a long time ago is create a domain alias for the license server. Now if the license server every changes between servers all I have to do is have the alias updated to the new servername or IP address. None of the clients even notice anything happened. I should have done that with the data and main archive servers as well, but I didn't think that far out.
I will probably at least get a dataserver alias when I eventually get the new servers to replace my old Win 2008 R2 servers. Hopefully IT can provide those soon.