Do I need to do anything particular to upgrade from 19.01, or can I just hit Install Updates and go for it?
Thanks much!
If you disable the sid repository you are good to go to update the base system to the latest Debian Buster base.
Like stated you will not get any theme updates however.
I just wanted to say, extremely belatedly, thanks. 
I’m really enjoying netrunner - it works beautifully for me. (I have a macbook pro 2012.)