I now have 3 version PE15, 2020 (18) and 2021 (19).
How do I pull all my projects from the previous versions into PE 2021 prior to uninstalling them?
We always recommend against doing it. It can corrupt the projects. Premiere Elements has never been designed to be backwards compatible, so we always recommend finishing your project in the same version it was started in.
But that said I've found newer versions of the program can usually open projects created by the generation before -- so you can likely open version 2020 projects in version 2021 and then save them as 2021 projects.
But there are no guarantees. So definitely make a copy of any project you're going to attempt this with. The older your project, the more likely it's going to bug something up.
I have to say I'm fairly shocked that this is the case. As a software professional for some decades, being able to bring old data forward into the current software was always a very high customer priority. And from the other side of the fence, as a Lightroom user for about a decade, I'm used to that being important too, for Adobe and its customers.
Perhaps both Adobe and its customers see videos as more ephemeral - "capture it, process it, publish it, move on and forget it" 🙂
There is Archive project option under file menu. First archive your projects using the version of PE you used to create the project and open that archived project incrementally. E.g. archive PE15 created project first, open this in PE18(2020), if opens fine, save and rearchive it and then open the same project in PE2021. You generally don't see any issue while opening a project created in 1 or 2 versions back.