Sorry, I never worried about the color scheme beause I always used the default one. There’s a vim file that keeps the color scheme in a .vimrc file in the home folder. I assume that this file was overwritten with the update.
But maybe you can configure the colors again, and make a copy of this file for the future. If it ever happens again, you will have a copy that you can use.
You can install new color schemes or even create your own.
But if you’ve never done this, chances are your old color scheme is still included as an available one. You can type
:colorscheme
then press space, then press TAB to show a list of installed schemes. Try them each.
One other reason I can think of for your scheme changing during an upgrade is maybe your vim now supports true color (24 bit color) and it didn’t before.
At any rate there’s a lot of flexibility in color schemes.