Officially, Premiere Elements is not backward compatible -- though sometimes it works. I've been able to open version 2018 projects in version 2019. But because it doesn't always work, we always recommend making a copy of any old project file before you try to open it with a newer version of the program. Just in case.
If you bought and registered version 2018, you can easily reinstall it. You can download it from here.
https://helpx.adobe.com/download-instal ... loads.htmlAnd you can recover your serial number by logging in at Adobe.com and clicking the Manage Account button. On the page that opens, click the Plans tab at the top of the screen. And, on the page that opens, click My Products. The serial numbers for all of your registered products will appear.