Install Catalina over Mojave not a clean install

[Mac Man 2007]

So I have forgotten how to do an install (not a clean install) using a USB stick with Catalina on it. Like always I have created a USB stick successfully using terminal via the Apple method using their terminal directions. I like having USBs around as I have a couple of laptops. I want to install Catalina on my wife's 2015 MacBook Pro presently running Mojave. I do NOT want to do a clean install. I just want to install over Mojave. What I do not remember is how to do it. I believe if I press the option key when starting the Mac with the thumb drive in it and start the process from there I will erase the contents resulting in a "clean install"?? Now I do have an external time machine for all my Macs just in case. But for her computer I just want to install Catalina over Mojave. I have been living in Mojave so long I forgot the exact process to just do an overwrite. I did a clean install of Mojave on my 2013 Mac air but I just do not need to do that with her computer. I know I sound stupids as @@@@ but I forgot the exact step. Duh
Thanks

 

[Rod]

A common use for a Bootable macOS Installer is to revert to a previous macOS. In order to overcome the Apple "barrier" which prevents retroactive macOS installation it is necessary to erase the HD at the Media level first BUT; in your case you are simply Upgrading to the next macOS (10.14 to 10.15) so you dont need to use a bootable installer at all. Really all you need to do is to download the macOS Catalina Installer directly from the App Store. Here is the link ‎macOS Catalina

 

[Rod]

PS. I should mention having bootable installers, at least for Intel Macs is a great idea. I used to do this a lot. At each new Upgrade I would make a copy of the macOS Installer then use it to create a bootable installer on a thumb drive. If the OS crashed or got irrevocably damaged or I simply wanted to go back after an Upgrade, together with a Time Machine backup I could erase and perform a clean instal of the previous OS and my data. All up it was a handy tool but that's all changed with Silicon Macs.

 

[Mac Man 2007]

thanks I already have it downloaded into my MacAir and made a bootable USB stick from that. So I should be able to use the USB stick without having to erase my HD in the MacBook Pro. I just need to be jogged as to the proper procedure. I understand I not not need a bootable installer but I have one. It takes too long to download it where I live. I should be able to upgrade from that USB. I just do not want to erase it. I believe if I press the option key and then install from there I will erase it. So there has to be a way to do it from this USB without erasing all the data? Or am I wrong the USB cannot just overwrite Mojave.

 

[Rod]

Mac Man, i cannot answer that question. I've never used a bootable installer to do a macOS Upgrade. Normally you would use Disk Utility on the USB stick to erase the HD first.
Using Terminal to create a Bootable installer changes some of the files in the original macOS installer (upgrader) app but it still retains the macOS installer's function.
So, theoretically it should work. Trial and error really. What if you don't boot from the USB, just open it on your desktop and if you see a macOS Catalina installer double click it and see if it gives you alternative locations for install. Obviously you want to install it on your Macintosh HD, not the USB.
I'm assuming you have a Time Machine backup just in case it goes sideways, we're in unchartered waters here.

 

[Mac Man 2007]

This worked like a charm. Just plug in the stick and click on it. Double click on installer and it gives me the drive and hit install. Perfect! I and do have a back up but this is working just fine. Thanks

 

[Rod]

Fantastic, we all learned something. Well done.