Upgrading elements 15 to 2020 and then upgrading to Catalina

Kim, as I said, I know very little about Macs.  But I don't imagine there are any unsolvable issues when you uninstall a prior version.  As for files that you have edited in prior versions, they will all show up and can be edited in the Elements Editor.  The one exception to this is that newer versions no longer support some varieties of pdf file. 

Kim, I know almost nothing about Macs.  However, I can tell you that Elements 15 will not work on a Catalina machine.

Each version of Elements is independent and two versions of Elements can be installed without bothering each other.  However, since version 15 won't work on your upgraded machine, there is no reason to keep it around.  Having said that, if you are using the Organizer, the existing catalog will be converted for use with the new version of Elements.  As part of the conversion process, it will make a backup copy of your old version 15 catalog.  However, once the conversion has taken place, you will not need to keep the backup copy (although the size is not very large, and it is always a good idea to keep multiple copies of your data files).

One final point: if you have not yet purchased Elements 2020, you may want to wait a little while since, based on past history, this is the time of year when a new version is likely to be released.  In the meantime, you can install a 30 day free trial of Elements 2020.