When making the transition to playing folk (or “trad”) flutes, there are a number of differences to account for. I tend to think of these in two categories: things that relate to the music theory of it all and things that relate to actually making the sound happen.
When considering the music theory side of things, it’s important to remember that you’re now in a different world. Imagine a fork in a road, way back in the day. Some parts of music went down the “classical” path, where music theory evolved and grew, and the customs and practices we use today were developed over time. Other parts of music went down the “folk/trad” path, where there was really very little change to the way things are done over time.
As a “classical” player dipping your toes into folk/trad flutes, you have to remember that some of the rules you know just don’t exist in that other world. I break this down into the categories of Instrument Key Naming and Transpositions and Notation. In general, these are something I would consider to be fundamental building blocks that are crucial to understand, in order to really feel comfortable with this new world of folk flutes.
When it comes to actually putting the flute up to your face and making sound come out, there are a few more categories to focus on. Each of these has its own way of contributing to your overall sound, and how authentic it is, and how much it honors the characteristic sound of that particular instrument. They are: fingering differences from Boehm-system flutes, ornamentation, tonguing, breathing, and vibrato.