How do you uninstall an older App after you've already moved to Catalina?

You asked how to uninstall them, not how to upgrade them

To upgrade them go to the source of the app and see what upgrades are available. Upgrade in the normal manner. Some will be paid upgrades, others will be zero charge.

It will depend how the applications were installed, and the nature of the vendor's uninstall solution. If the uninstaller is itself a 32-bit application, then it won't work in Catalina. The vendor will have to address making a 64-bit uninstaller application, and that would be you contacting the vendor and determining if they have one available.

If the application has the ability to uninstall itself, whether by its own menu item, or by running a shell script from the Terminal, then that might work too.

Some applications only install in /Applications, and you know this because their installers typically show illustrate dragging the application onto an application folder alias (e.g. LibreOffice).

Application installers that request your administrator's password to install typically get shot into the areas of the operating system, and without a proper uninstall tool, they can be problematic in locating all of the components of the application.

Most applications also write to folders in /Users/$USERNAME/Library/Application Support, and those should be considered as part of the application removal process.

shshapero wrote:

Thanks. Yeah, I have an upgrade dmg file from the vendor, but part of the recommendation is to uninstall the previous version. I was just wondering if you were able to run your newer software deleting the dead apps and emptying the trash. I'm sure it's kind of a case by case basis.

There was only one I cared about, and I just ran the upgrade. There rest were long overdue for weeding out

I leaped before I looked, too. I had no idea how many apps were affected. All I did was deleted the dead apps (moved to Trash) and emptied Trash. That was all I could find to do