You may have had the same reaction I did when I first started loading up the Develop module in Adobe's new Lightroom Classic CC update: oh no, why did Lightroom change all of my photos to an old process version!? First things first: don't worry. The process version did not change. Instead, Lightroom Classic CC introduces the fourth iteration of these so-called process versions, but this one (the first in half a decade) is different for a number of reasons.
First and foremost, we're used to these process versions being a big deal. They don't change often, and the change between the 2010 and 2012 versions didn't shift colors to completely opposite ends of the spectrum. But there was certainly a visible change to each image if all one did was to update the process version of an edited image. But since 2012, we've actually been using the same 2012 process. You're still right to be worried once again about another process, however, especially after all of this time. There must be a number of visible changes now, right?
In fact, Process Version 4 simply introduces support for Lightroom Classic CC's new masking features such as the Color and Luminance Range masks. You can feel free to update your photos from Process Version 3 (2012) to Version 4 without a noticeable change in the edited image. Likewise, you can leave it alone, and each file will update on its own if you use any of the new masking features. It's up to you.
If you do want to update the file or are wondering about which process version you're using, take a look right below the histogram window at the bottom right while in the Develop module. If you see a small lightning bolt symbol, that photograph is set to use an older process version (either 2012 or earlier). Upon clicking on the lightning bolt icon, Lightroom makes it easy to apply the change to all of the photographs in your current Filmstrip if you so desire by prompting you with a choice to change just that image or all of the ones in the current Filmstrip. You can also see the current process version and select from any previous version as the first option in the Camera Calibration dropdown, also in the Develop module.
Keep calm, edit on.