How to change default markup?

1

To use mmark markup, I need to specify markup: "mmark" in frontmatter or change extension name.

But is it possible to change default option with md file?

2

The default markup depends on the file extension. If you have a large number of files, you need to use a special program for mass renaming (Ant Renamer, Far Manager etc.), or write a small script for mass renaming.