[tom_]

It definitely switches. There's four 1MHz timers and a vsync interrupt, so this stuff is very easy to arrange and the timing doesn't have to be very precise. (Ideally you'd leave more of a gap than Elite did, though, because it takes basically an entire scanline to do the mode switch and reprogram the palette.)

Try it on an emulator: https://bbc.godbolt.org/ - Elite is the default disc. To load: hold Shift, tap F12, release shift. Once the game is loaded and you see the animating ship, hold F12. The mode-switching interrupt stops running while it's held down (that key is wired to the reset pin on the real hardware), and you'll see the entire display incorrectly scanned out in just one of the modes.