I am currently using an ATEM Mini Pro, and I would like to use have my lower thirds on while changing cameras. I am wondering if it's possible and what I should be doing. Thank you

There is a setting for this (it affects upstream keyer only) in the ATEM Setup software app. Connect your ATEM to the PC via USB, launch the setup app and select your ATEM. Then make the configuration change.

Alternatively use the Down Stream Keyer

You should have your lower third graphic in a format that allows transparency, e.g. PNG, so that the upper two thirds is transparent. Load it into the media pool in ATEM software control using the media page selected using the button at the bottom of the window. Blackmagic provide some sample graphics if you want to try out known working ones.

Return back to the Switcher page and make sure Palettes is selected top right column. Scroll down column until you reach Downsteam key. Select Media Player 1 as the Fill source, and Media Player 1 Key as the key source. Make sure the Pre Multiplied Key button is on.

Just need to pick the correct graphic from the Media Player if you have more than one loaded. At the top of the right hand column click on Media Player. In the MP1 section click on the Media menu and select the file name of your choice.

Now to put it on screen. Just to the left of the rightmost column is a section titled DSK1. Click on the On Air button so it glows red. Your lower third graphic should now show, and stay up as you switch camera. When you get tired of it just click on the On Air button again to turn it off.

If you would like your lower third graphic to fade in and out instead of instantly appearing and disappearing, put a number in seconds in the rate box and hit the Auto button instead.

Tim