Premiere Elements 2018 compatibility with prior versions.

That is not a safe assumption.  There is no promise that the new will work with the old. 

The problem is that features of the old may be abandon or exchanged for features in the new.  A very simple project with no corrections, adjustments, transitions might work. 

The good news is there are workarounds.

First, the two versions can stay on your computer and work independently.  They will not interfere with each other. 

Second, you can output a best quality version from the old version and use it as media in the new version. 

If you do decide to open an old project in a newer version please be sure you are doing it with project copies.  The new version can actually corrupt a file from and old version.  Use the file menu and do a Save As... and name it something like "Project file for new version".  Then if something goes wrong you've lost nothing.

Bill