Never mind, it turns out:

1. I mistakenly thought the "TimeSig" field sandwiched between "Feel" and "key" indicated the time signature *of the song* not (as the mouseOver hint I didn't stop to read says) the "preferred time signature of the style." That's what I clicked, and I was surprised to see no changes after I selected 2/4 from the dialog that opens when you click it. (At this point, I'm not sure what that dialog does, to be frank, but that's for another day).

2. Right clicking on the bars (or F5) and changing the number of beats per bar to 2 is indeed the way to go, but I thought it failed because I was expecting the "TimeSig" filed mentioned above to change as well, which it didn't. The only visible effect is the appearance of little red lines in each bar, whose meaning escapes me at the moment.

3. Apparently, and to the best of my attempts, the 2 lead-in bars stay in 4/4 even if you managed to bring the song itself to 2/4. I may be wrong here, but haven't found a way to fix that.

The lack of visual feedback (point 2) plus the mistaken lead in (point 3), led me think I could not change the meter. My bad. I now export without the lead-in bars and the problem is solved.

(Oh, and I am on BIAB 2019)