I believe this is controlled from the Server Administration GUI. My understanding is that the file-based SMAT is being deprecated, so you may want to consider staying with the SMAT DB as this is the standard going forward.
Does not appear to be an option under server administrator at least on 19.1 with Windows. I am aware of that direction e, but there are other scripts that run that depend on the smat files at least for now.
I would use a “replicate and rename” approach if you wanted to modify an existing site by changing the SMAT setup from SMATDB to SMAT files. Basically, create a new SMAT file-based site and “load” that new site with the old site configuration. (there are many ways to do this). Then rename the two sites appropriately.
If your goal were to provide SMATDB data to an external process that requires SMAT Files, I would create a conversion process that converts the specific SMAT DB to a set of SMAT Files and then serves the SMAT Files to your external process.
Stay with SMATDB and create new processes to appease old legacy needs if you can.