Upgrading from inside Mojave to Catalina query.
If i upgrade from MacOs mojave to catalina will it wipe out my mojave installation? Or is it ok to run the update from within Mojave?
Earlier Mac models
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When you upgrade to a new(er) version of macOS (for example, from Mojave to Catalina), it will replace the existing version of macOS. So, yes, your Mojave installation would be replaced by Catalina. The upgrade replaces the version of macOS. It does not wipe or replace your apps, data or users.
That said, you should make at least one complete backup of all your data to an external disk before upgrading macOS.
And be aware that if you have any older "32-bit" apps on Mojave they will not run on Catalina or later versions of macOS. Starting with Catalina all apps must be "64-bit" apps. This article shows you two ways to check your apps in advance -> https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html