- Is it possible to convert a whole project to use .tscn instead of .scn?
No, you will need to save each scene (
.scn) and resource (
.res) to their respective text formats,
.tres. You will also have to remove the old scene/resource files after converting, and make sure all dependencies are still met.
- is there a way to set .tscn as the default in the "save as" dialog?
Not that I know of.